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# 2.4   Space Complexity

<u>Space complexity</u> measures the growth trend of memory space occupied by an algorithm as the data size increases. This concept is very similar to time complexity, except that "running time" is replaced with "occupied memory space".

## 2.4.1 &nbsp; Algorithm-Related Space

The memory space used by an algorithm during execution mainly includes the following types.

- **Input space**: Used to store the input data of the algorithm.
- **Temporary space**: Used to store variables, objects, function contexts, and other data during the algorithm's execution.
- **Output space**: Used to store the output data of the algorithm.

In general, the scope of space complexity statistics is "temporary space" plus "output space".

Temporary space can be further divided into three parts.

- **Temporary data**: Used to save various constants, variables, objects, etc., during the algorithm's execution.
- **Stack frame space**: Used to save the context data of called functions. The system creates a stack frame at the top of the stack each time a function is called, and the stack frame space is released after the function returns.
- **Instruction space**: Used to save compiled program instructions, which are usually ignored in actual statistics.

When analyzing the space complexity of a program, **we usually consider three parts: temporary data, stack frame space, and output data**, as shown in the following figure.

![Algorithm-related space](space_complexity.assets/space_types.png){ class="animation-figure" }

<p align="center"> Figure 2-15 &nbsp; Algorithm-related space </p>

The related code is as follows:

=== "Python"

    ```python title=""
    class Node:
        """Class"""
        def __init__(self, x: int):
            self.val: int = x              # Node value
            self.next: Node | None = None  # Reference to the next node

    def function() -> int:
        """Function"""
        # Perform some operations...
        return 0

    def algorithm(n) -> int:  # Input data
        A = 0                 # Temporary data (constant, usually represented by uppercase letters)
        b = 0                 # Temporary data (variable)
        node = Node(0)        # Temporary data (object)
        c = function()        # Stack frame space (function call)
        return A + b + c      # Output data
    ```

=== "C++"

    ```cpp title=""
    /* Structure */
    struct Node {
        int val;
        Node *next;
        Node(int x) : val(x), next(nullptr) {}
    };

    /* Function */
    int func() {
        // Perform some operations...
        return 0;
    }

    int algorithm(int n) {        // Input data
        const int a = 0;          // Temporary data (constant)
        int b = 0;                // Temporary data (variable)
        Node* node = new Node(0); // Temporary data (object)
        int c = func();           // Stack frame space (function call)
        return a + b + c;         // Output data
    }
    ```

=== "Java"

    ```java title=""
    /* Class */
    class Node {
        int val;
        Node next;
        Node(int x) { val = x; }
    }

    /* Function */
    int function() {
        // Perform some operations...
        return 0;
    }

    int algorithm(int n) {        // Input data
        final int a = 0;          // Temporary data (constant)
        int b = 0;                // Temporary data (variable)
        Node node = new Node(0);  // Temporary data (object)
        int c = function();       // Stack frame space (function call)
        return a + b + c;         // Output data
    }
    ```

=== "C#"

    ```csharp title=""
    /* Class */
    class Node(int x) {
        int val = x;
        Node next;
    }

    /* Function */
    int Function() {
        // Perform some operations...
        return 0;
    }

    int Algorithm(int n) {        // Input data
        const int a = 0;          // Temporary data (constant)
        int b = 0;                // Temporary data (variable)
        Node node = new(0);       // Temporary data (object)
        int c = Function();       // Stack frame space (function call)
        return a + b + c;         // Output data
    }
    ```

=== "Go"

    ```go title=""
    /* Structure */
    type node struct {
        val  int
        next *node
    }

    /* Create node structure */
    func newNode(val int) *node {
        return &node{val: val}
    }

    /* Function */
    func function() int {
        // Perform some operations...
        return 0
    }

    func algorithm(n int) int { // Input data
        const a = 0             // Temporary data (constant)
        b := 0                  // Temporary data (variable)
        newNode(0)              // Temporary data (object)
        c := function()         // Stack frame space (function call)
        return a + b + c        // Output data
    }
    ```

=== "Swift"

    ```swift title=""
    /* Class */
    class Node {
        var val: Int
        var next: Node?

        init(x: Int) {
            val = x
        }
    }

    /* Function */
    func function() -> Int {
        // Perform some operations...
        return 0
    }

    func algorithm(n: Int) -> Int { // Input data
        let a = 0             // Temporary data (constant)
        var b = 0             // Temporary data (variable)
        let node = Node(x: 0) // Temporary data (object)
        let c = function()    // Stack frame space (function call)
        return a + b + c      // Output data
    }
    ```

=== "JS"

    ```javascript title=""
    /* Class */
    class Node {
        val;
        next;
        constructor(val) {
            this.val = val === undefined ? 0 : val; // Node value
            this.next = null;                       // Reference to the next node
        }
    }

    /* Function */
    function constFunc() {
        // Perform some operations
        return 0;
    }

    function algorithm(n) {       // Input data
        const a = 0;              // Temporary data (constant)
        let b = 0;                // Temporary data (variable)
        const node = new Node(0); // Temporary data (object)
        const c = constFunc();    // Stack frame space (function call)
        return a + b + c;         // Output data
    }
    ```

=== "TS"

    ```typescript title=""
    /* Class */
    class Node {
        val: number;
        next: Node | null;
        constructor(val?: number) {
            this.val = val === undefined ? 0 : val; // Node value
            this.next = null;                       // Reference to the next node
        }
    }

    /* Function */
    function constFunc(): number {
        // Perform some operations
        return 0;
    }

    function algorithm(n: number): number { // Input data
        const a = 0;                        // Temporary data (constant)
        let b = 0;                          // Temporary data (variable)
        const node = new Node(0);           // Temporary data (object)
        const c = constFunc();              // Stack frame space (function call)
        return a + b + c;                   // Output data
    }
    ```

=== "Dart"

    ```dart title=""
    /* Class */
    class Node {
      int val;
      Node next;
      Node(this.val, [this.next]);
    }

    /* Function */
    int function() {
      // Perform some operations...
      return 0;
    }

    int algorithm(int n) {  // Input data
      const int a = 0;      // Temporary data (constant)
      int b = 0;            // Temporary data (variable)
      Node node = Node(0);  // Temporary data (object)
      int c = function();   // Stack frame space (function call)
      return a + b + c;     // Output data
    }
    ```

=== "Rust"

    ```rust title=""
    use std::rc::Rc;
    use std::cell::RefCell;

    /* Structure */
    struct Node {
        val: i32,
        next: Option<Rc<RefCell<Node>>>,
    }

    /* Create Node structure */
    impl Node {
        fn new(val: i32) -> Self {
            Self { val: val, next: None }
        }
    }

    /* Function */
    fn function() -> i32 {
        // Perform some operations...
        return 0;
    }

    fn algorithm(n: i32) -> i32 {       // Input data
        const a: i32 = 0;               // Temporary data (constant)
        let mut b = 0;                  // Temporary data (variable)
        let node = Node::new(0);        // Temporary data (object)
        let c = function();             // Stack frame space (function call)
        return a + b + c;               // Output data
    }
    ```

=== "C"

    ```c title=""
    /* Function */
    int func() {
        // Perform some operations...
        return 0;
    }

    int algorithm(int n) { // Input data
        const int a = 0;   // Temporary data (constant)
        int b = 0;         // Temporary data (variable)
        int c = func();    // Stack frame space (function call)
        return a + b + c;  // Output data
    }
    ```

=== "Kotlin"

    ```kotlin title=""
    /* Class */
    class Node(var _val: Int) {
        var next: Node? = null
    }

    /* Function */
    fun function(): Int {
        // Perform some operations...
        return 0
    }

    fun algorithm(n: Int): Int { // Input data
        val a = 0                // Temporary data (constant)
        var b = 0                // Temporary data (variable)
        val node = Node(0)       // Temporary data (object)
        val c = function()       // Stack frame space (function call)
        return a + b + c         // Output data
    }
    ```

=== "Ruby"

    ```ruby title=""
    ### Class ###
    class Node
        attr_accessor :val      # Node value
        attr_accessor :next     # Reference to the next node

        def initialize(x)
            @val = x
        end
    end

    ### Function ###
    def function
        # Perform some operations...
        0
    end

    ### Algorithm ###
    def algorithm(n)        # Input data
        a = 0               # Temporary data (constant)
        b = 0               # Temporary data (variable)
        node = Node.new(0)  # Temporary data (object)
        c = function        # Stack frame space (function call)
        a + b + c           # Output data
    end
    ```

## 2.4.2 &nbsp; Calculation Method

The calculation method for space complexity is roughly the same as for time complexity, except that what we measure changes from the "number of operations" to the "amount of space used".

Unlike time complexity, **we usually only focus on the worst-case space complexity**. This is because memory space is a hard requirement, and we must ensure that sufficient memory space is reserved for all input data.

Observe the following code. Here, "worst case" in worst-case space complexity has two meanings.

1. **Based on the worst input data**: When $n < 10$, the space complexity is $O(1)$; but when $n > 10$, the initialized array `nums` occupies $O(n)$ space, so the worst-case space complexity is $O(n)$.
2. **Based on the peak memory during algorithm execution**: For example, before executing the last line, the program occupies $O(1)$ space; when initializing the array `nums`, the program occupies $O(n)$ space, so the worst-case space complexity is $O(n)$.

=== "Python"

    ```python title=""
    def algorithm(n: int):
        a = 0               # O(1)
        b = [0] * 10000     # O(1)
        if n > 10:
            nums = [0] * n  # O(n)
    ```

=== "C++"

    ```cpp title=""
    void algorithm(int n) {
        int a = 0;               // O(1)
        vector<int> b(10000);    // O(1)
        if (n > 10)
            vector<int> nums(n); // O(n)
    }
    ```

=== "Java"

    ```java title=""
    void algorithm(int n) {
        int a = 0;                   // O(1)
        int[] b = new int[10000];    // O(1)
        if (n > 10)
            int[] nums = new int[n]; // O(n)
    }
    ```

=== "C#"

    ```csharp title=""
    void Algorithm(int n) {
        int a = 0;                   // O(1)
        int[] b = new int[10000];    // O(1)
        if (n > 10) {
            int[] nums = new int[n]; // O(n)
        }
    }
    ```

=== "Go"

    ```go title=""
    func algorithm(n int) {
        a := 0                      // O(1)
        b := make([]int, 10000)     // O(1)
        var nums []int
        if n > 10 {
            nums := make([]int, n)  // O(n)
        }
        fmt.Println(a, b, nums)
    }
    ```

=== "Swift"

    ```swift title=""
    func algorithm(n: Int) {
        let a = 0 // O(1)
        let b = Array(repeating: 0, count: 10000) // O(1)
        if n > 10 {
            let nums = Array(repeating: 0, count: n) // O(n)
        }
    }
    ```

=== "JS"

    ```javascript title=""
    function algorithm(n) {
        const a = 0;                   // O(1)
        const b = new Array(10000);    // O(1)
        if (n > 10) {
            const nums = new Array(n); // O(n)
        }
    }
    ```

=== "TS"

    ```typescript title=""
    function algorithm(n: number): void {
        const a = 0;                   // O(1)
        const b = new Array(10000);    // O(1)
        if (n > 10) {
            const nums = new Array(n); // O(n)
        }
    }
    ```

=== "Dart"

    ```dart title=""
    void algorithm(int n) {
      int a = 0;                            // O(1)
      List<int> b = List.filled(10000, 0);  // O(1)
      if (n > 10) {
        List<int> nums = List.filled(n, 0); // O(n)
      }
    }
    ```

=== "Rust"

    ```rust title=""
    fn algorithm(n: i32) {
        let a = 0;                              // O(1)
        let b = [0; 10000];                     // O(1)
        if n > 10 {
            let nums = vec![0; n as usize];     // O(n)
        }
    }
    ```

=== "C"

    ```c title=""
    void algorithm(int n) {
        int a = 0;               // O(1)
        int b[10000];            // O(1)
        if (n > 10)
            int nums[n] = {0};   // O(n)
    }
    ```

=== "Kotlin"

    ```kotlin title=""
    fun algorithm(n: Int) {
        val a = 0                    // O(1)
        val b = IntArray(10000)      // O(1)
        if (n > 10) {
            val nums = IntArray(n)   // O(n)
        }
    }
    ```

=== "Ruby"

    ```ruby title=""
    def algorithm(n)
        a = 0                           # O(1)
        b = Array.new(10000)            # O(1)
        nums = Array.new(n) if n > 10   # O(n)
    end
    ```

**In recursive functions, it is necessary to count the stack frame space**. Observe the following code:

=== "Python"

    ```python title=""
    def function() -> int:
        # Perform some operations
        return 0

    def loop(n: int):
        """Loop has space complexity of O(1)"""
        for _ in range(n):
            function()

    def recur(n: int):
        """Recursion has space complexity of O(n)"""
        if n == 1:
            return
        return recur(n - 1)
    ```

=== "C++"

    ```cpp title=""
    int func() {
        // Perform some operations
        return 0;
    }
    /* Loop has space complexity of O(1) */
    void loop(int n) {
        for (int i = 0; i < n; i++) {
            func();
        }
    }
    /* Recursion has space complexity of O(n) */
    void recur(int n) {
        if (n == 1) return;
        recur(n - 1);
    }
    ```

=== "Java"

    ```java title=""
    int function() {
        // Perform some operations
        return 0;
    }
    /* Loop has space complexity of O(1) */
    void loop(int n) {
        for (int i = 0; i < n; i++) {
            function();
        }
    }
    /* Recursion has space complexity of O(n) */
    void recur(int n) {
        if (n == 1) return;
        recur(n - 1);
    }
    ```

=== "C#"

    ```csharp title=""
    int Function() {
        // Perform some operations
        return 0;
    }
    /* Loop has space complexity of O(1) */
    void Loop(int n) {
        for (int i = 0; i < n; i++) {
            Function();
        }
    }
    /* Recursion has space complexity of O(n) */
    int Recur(int n) {
        if (n == 1) return 1;
        return Recur(n - 1);
    }
    ```

=== "Go"

    ```go title=""
    func function() int {
        // Perform some operations
        return 0
    }

    /* Loop has space complexity of O(1) */
    func loop(n int) {
        for i := 0; i < n; i++ {
            function()
        }
    }

    /* Recursion has space complexity of O(n) */
    func recur(n int) {
        if n == 1 {
            return
        }
        recur(n - 1)
    }
    ```

=== "Swift"

    ```swift title=""
    @discardableResult
    func function() -> Int {
        // Perform some operations
        return 0
    }

    /* Loop has space complexity of O(1) */
    func loop(n: Int) {
        for _ in 0 ..< n {
            function()
        }
    }

    /* Recursion has space complexity of O(n) */
    func recur(n: Int) {
        if n == 1 {
            return
        }
        recur(n: n - 1)
    }
    ```

=== "JS"

    ```javascript title=""
    function constFunc() {
        // Perform some operations
        return 0;
    }
    /* Loop has space complexity of O(1) */
    function loop(n) {
        for (let i = 0; i < n; i++) {
            constFunc();
        }
    }
    /* Recursion has space complexity of O(n) */
    function recur(n) {
        if (n === 1) return;
        return recur(n - 1);
    }
    ```

=== "TS"

    ```typescript title=""
    function constFunc(): number {
        // Perform some operations
        return 0;
    }
    /* Loop has space complexity of O(1) */
    function loop(n: number): void {
        for (let i = 0; i < n; i++) {
            constFunc();
        }
    }
    /* Recursion has space complexity of O(n) */
    function recur(n: number): void {
        if (n === 1) return;
        return recur(n - 1);
    }
    ```

=== "Dart"

    ```dart title=""
    int function() {
      // Perform some operations
      return 0;
    }
    /* Loop has space complexity of O(1) */
    void loop(int n) {
      for (int i = 0; i < n; i++) {
        function();
      }
    }
    /* Recursion has space complexity of O(n) */
    void recur(int n) {
      if (n == 1) return;
      recur(n - 1);
    }
    ```

=== "Rust"

    ```rust title=""
    fn function() -> i32 {
        // Perform some operations
        return 0;
    }
    /* Loop has space complexity of O(1) */
    fn loop(n: i32) {
        for i in 0..n {
            function();
        }
    }
    /* Recursion has space complexity of O(n) */
    fn recur(n: i32) {
        if n == 1 {
            return;
        }
        recur(n - 1);
    }
    ```

=== "C"

    ```c title=""
    int func() {
        // Perform some operations
        return 0;
    }
    /* Loop has space complexity of O(1) */
    void loop(int n) {
        for (int i = 0; i < n; i++) {
            func();
        }
    }
    /* Recursion has space complexity of O(n) */
    void recur(int n) {
        if (n == 1) return;
        recur(n - 1);
    }
    ```

=== "Kotlin"

    ```kotlin title=""
    fun function(): Int {
        // Perform some operations
        return 0
    }
    /* Loop has space complexity of O(1) */
    fun loop(n: Int) {
        for (i in 0..<n) {
            function()
        }
    }
    /* Recursion has space complexity of O(n) */
    fun recur(n: Int) {
        if (n == 1) return
        return recur(n - 1)
    }
    ```

=== "Ruby"

    ```ruby title=""
    def function
        # Perform some operations
        0
    end

    ### Loop has space complexity of O(1) ###
    def loop(n)
        (0...n).each { function }
    end

    ### Recursion has space complexity of O(n) ###
    def recur(n)
        return if n == 1
        recur(n - 1)
    end
    ```

The time complexity of both functions `loop()` and `recur()` is $O(n)$, but their space complexities are different.

- The function `loop()` calls `function()` $n$ times in a loop. In each iteration, `function()` returns and releases its stack frame space, so the space complexity remains $O(1)$.
- The recursive function `recur()` has $n$ unreturned `recur()` instances existing simultaneously during execution, thus occupying $O(n)$ stack frame space.

## 2.4.3 &nbsp; Common Types

Let the input data size be $n$. The following figure shows common types of space complexity (arranged from low to high).

$$
\begin{aligned}
& O(1) < O(\log n) < O(n) < O(n^2) < O(2^n) \newline
& \text{Constant} < \text{Logarithmic} < \text{Linear} < \text{Quadratic} < \text{Exponential}
\end{aligned}
$$

![Common types of space complexity](space_complexity.assets/space_complexity_common_types.png){ class="animation-figure" }

<p align="center"> Figure 2-16 &nbsp; Common types of space complexity </p>

### 1. &nbsp; Constant Order $O(1)$ {data-toc-label="1. &nbsp; Constant Order"}

Constant order is common for constants, variables, and objects whose number is independent of the input data size $n$.

It should be noted that memory occupied by initializing variables or calling functions in a loop is released when entering the next iteration, so it does not accumulate space, and the space complexity remains $O(1)$:

=== "Python"

    ```python title="space_complexity.py"
    def function() -> int:
        """Function"""
        # Perform some operations
        return 0

    def constant(n: int):
        """Constant order"""
        # Constants, variables, objects occupy O(1) space
        a = 0
        nums = [0] * 10000
        node = ListNode(0)
        # Variables in the loop occupy O(1) space
        for _ in range(n):
            c = 0
        # Functions in the loop occupy O(1) space
        for _ in range(n):
            function()
    ```

=== "C++"

    ```cpp title="space_complexity.cpp"
    /* Function */
    int func() {
        // Perform some operations
        return 0;
    }

    /* Constant order */
    void constant(int n) {
        // Constants, variables, objects occupy O(1) space
        const int a = 0;
        int b = 0;
        vector<int> nums(10000);
        ListNode node(0);
        // Variables in the loop occupy O(1) space
        for (int i = 0; i < n; i++) {
            int c = 0;
        }
        // Functions in the loop occupy O(1) space
        for (int i = 0; i < n; i++) {
            func();
        }
    }
    ```

=== "Java"

    ```java title="space_complexity.java"
    /* Function */
    int function() {
        // Perform some operations
        return 0;
    }

    /* Constant order */
    void constant(int n) {
        // Constants, variables, objects occupy O(1) space
        final int a = 0;
        int b = 0;
        int[] nums = new int[10000];
        ListNode node = new ListNode(0);
        // Variables in the loop occupy O(1) space
        for (int i = 0; i < n; i++) {
            int c = 0;
        }
        // Functions in the loop occupy O(1) space
        for (int i = 0; i < n; i++) {
            function();
        }
    }
    ```

=== "C#"

    ```csharp title="space_complexity.cs"
    /* Function */
    int Function() {
        // Perform some operations
        return 0;
    }

    /* Constant order */
    void Constant(int n) {
        // Constants, variables, objects occupy O(1) space
        int a = 0;
        int b = 0;
        int[] nums = new int[10000];
        ListNode node = new(0);
        // Variables in the loop occupy O(1) space
        for (int i = 0; i < n; i++) {
            int c = 0;
        }
        // Functions in the loop occupy O(1) space
        for (int i = 0; i < n; i++) {
            Function();
        }
    }
    ```

=== "Go"

    ```go title="space_complexity.go"
    /* Function */
    func function() int {
        // Perform some operations...
        return 0
    }

    /* Constant order */
    func spaceConstant(n int) {
        // Constants, variables, objects occupy O(1) space
        const a = 0
        b := 0
        nums := make([]int, 10000)
        node := newNode(0)
        // Variables in the loop occupy O(1) space
        var c int
        for i := 0; i < n; i++ {
            c = 0
        }
        // Functions in the loop occupy O(1) space
        for i := 0; i < n; i++ {
            function()
        }
        b += 0
        c += 0
        nums[0] = 0
        node.val = 0
    }
    ```

=== "Swift"

    ```swift title="space_complexity.swift"
    /* Function */
    @discardableResult
    func function() -> Int {
        // Perform some operations
        return 0
    }

    /* Constant order */
    func constant(n: Int) {
        // Constants, variables, objects occupy O(1) space
        let a = 0
        var b = 0
        let nums = Array(repeating: 0, count: 10000)
        let node = ListNode(x: 0)
        // Variables in the loop occupy O(1) space
        for _ in 0 ..< n {
            let c = 0
        }
        // Functions in the loop occupy O(1) space
        for _ in 0 ..< n {
            function()
        }
    }
    ```

=== "JS"

    ```javascript title="space_complexity.js"
    /* Function */
    function constFunc() {
        // Perform some operations
        return 0;
    }

    /* Constant order */
    function constant(n) {
        // Constants, variables, objects occupy O(1) space
        const a = 0;
        const b = 0;
        const nums = new Array(10000);
        const node = new ListNode(0);
        // Variables in the loop occupy O(1) space
        for (let i = 0; i < n; i++) {
            const c = 0;
        }
        // Functions in the loop occupy O(1) space
        for (let i = 0; i < n; i++) {
            constFunc();
        }
    }
    ```

=== "TS"

    ```typescript title="space_complexity.ts"
    /* Function */
    function constFunc(): number {
        // Perform some operations
        return 0;
    }

    /* Constant order */
    function constant(n: number): void {
        // Constants, variables, objects occupy O(1) space
        const a = 0;
        const b = 0;
        const nums = new Array(10000);
        const node = new ListNode(0);
        // Variables in the loop occupy O(1) space
        for (let i = 0; i < n; i++) {
            const c = 0;
        }
        // Functions in the loop occupy O(1) space
        for (let i = 0; i < n; i++) {
            constFunc();
        }
    }
    ```

=== "Dart"

    ```dart title="space_complexity.dart"
    /* Function */
    int function() {
      // Perform some operations
      return 0;
    }

    /* Constant order */
    void constant(int n) {
      // Constants, variables, objects occupy O(1) space
      final int a = 0;
      int b = 0;
      List<int> nums = List.filled(10000, 0);
      ListNode node = ListNode(0);
      // Variables in the loop occupy O(1) space
      for (var i = 0; i < n; i++) {
        int c = 0;
      }
      // Functions in the loop occupy O(1) space
      for (var i = 0; i < n; i++) {
        function();
      }
    }
    ```

=== "Rust"

    ```rust title="space_complexity.rs"
    /* Function */
    fn function() -> i32 {
        // Perform some operations
        return 0;
    }

    /* Constant order */
    #[allow(unused)]
    fn constant(n: i32) {
        // Constants, variables, objects occupy O(1) space
        const A: i32 = 0;
        let b = 0;
        let nums = vec![0; 10000];
        let node = ListNode::new(0);
        // Variables in the loop occupy O(1) space
        for i in 0..n {
            let c = 0;
        }
        // Functions in the loop occupy O(1) space
        for i in 0..n {
            function();
        }
    }
    ```

=== "C"

    ```c title="space_complexity.c"
    /* Function */
    int func() {
        // Perform some operations
        return 0;
    }

    /* Constant order */
    void constant(int n) {
        // Constants, variables, objects occupy O(1) space
        const int a = 0;
        int b = 0;
        int nums[1000];
        ListNode *node = newListNode(0);
        free(node);
        // Variables in the loop occupy O(1) space
        for (int i = 0; i < n; i++) {
            int c = 0;
        }
        // Functions in the loop occupy O(1) space
        for (int i = 0; i < n; i++) {
            func();
        }
    }
    ```

=== "Kotlin"

    ```kotlin title="space_complexity.kt"
    /* Function */
    fun function(): Int {
        // Perform some operations
        return 0
    }

    /* Constant order */
    fun constant(n: Int) {
        // Constants, variables, objects occupy O(1) space
        val a = 0
        var b = 0
        val nums = Array(10000) { 0 }
        val node = ListNode(0)
        // Variables in the loop occupy O(1) space
        for (i in 0..<n) {
            val c = 0
        }
        // Functions in the loop occupy O(1) space
        for (i in 0..<n) {
            function()
        }
    }
    ```

=== "Ruby"

    ```ruby title="space_complexity.rb"
    ### Function ###
    def function
      # Perform some operations
      0
    end

    ### Constant time ###
    def constant(n)
      # Constants, variables, objects occupy O(1) space
      a = 0
      nums = [0] * 10000
      node = ListNode.new

      # Variables in the loop occupy O(1) space
      (0...n).each { c = 0 }
      # Functions in the loop occupy O(1) space
      (0...n).each { function }
    end
    ```

### 2. &nbsp; Linear Order $O(n)$ {data-toc-label="2. &nbsp; Linear Order"}

Linear order is common in arrays, linked lists, stacks, queues, etc., where the number of elements is proportional to $n$:

=== "Python"

    ```python title="space_complexity.py"
    def linear(n: int):
        """Linear order"""
        # A list of length n occupies O(n) space
        nums = [0] * n
        # A hash table of length n occupies O(n) space
        hmap = dict[int, str]()
        for i in range(n):
            hmap[i] = str(i)
    ```

=== "C++"

    ```cpp title="space_complexity.cpp"
    /* Linear order */
    void linear(int n) {
        // Array of length n uses O(n) space
        vector<int> nums(n);
        // A list of length n occupies O(n) space
        vector<ListNode> nodes;
        for (int i = 0; i < n; i++) {
            nodes.push_back(ListNode(i));
        }
        // A hash table of length n occupies O(n) space
        unordered_map<int, string> map;
        for (int i = 0; i < n; i++) {
            map[i] = to_string(i);
        }
    }
    ```

=== "Java"

    ```java title="space_complexity.java"
    /* Linear order */
    void linear(int n) {
        // Array of length n uses O(n) space
        int[] nums = new int[n];
        // A list of length n occupies O(n) space
        List<ListNode> nodes = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            nodes.add(new ListNode(i));
        }
        // A hash table of length n occupies O(n) space
        Map<Integer, String> map = new HashMap<>();
        for (int i = 0; i < n; i++) {
            map.put(i, String.valueOf(i));
        }
    }
    ```

=== "C#"

    ```csharp title="space_complexity.cs"
    /* Linear order */
    void Linear(int n) {
        // Array of length n uses O(n) space
        int[] nums = new int[n];
        // A list of length n occupies O(n) space
        List<ListNode> nodes = [];
        for (int i = 0; i < n; i++) {
            nodes.Add(new ListNode(i));
        }
        // A hash table of length n occupies O(n) space
        Dictionary<int, string> map = [];
        for (int i = 0; i < n; i++) {
            map.Add(i, i.ToString());
        }
    }
    ```

=== "Go"

    ```go title="space_complexity.go"
    /* Linear order */
    func spaceLinear(n int) {
        // Array of length n uses O(n) space
        _ = make([]int, n)
        // A list of length n occupies O(n) space
        var nodes []*node
        for i := 0; i < n; i++ {
            nodes = append(nodes, newNode(i))
        }
        // A hash table of length n occupies O(n) space
        m := make(map[int]string, n)
        for i := 0; i < n; i++ {
            m[i] = strconv.Itoa(i)
        }
    }
    ```

=== "Swift"

    ```swift title="space_complexity.swift"
    /* Linear order */
    func linear(n: Int) {
        // Array of length n uses O(n) space
        let nums = Array(repeating: 0, count: n)
        // A list of length n occupies O(n) space
        let nodes = (0 ..< n).map { ListNode(x: $0) }
        // A hash table of length n occupies O(n) space
        let map = Dictionary(uniqueKeysWithValues: (0 ..< n).map { ($0, "\($0)") })
    }
    ```

=== "JS"

    ```javascript title="space_complexity.js"
    /* Linear order */
    function linear(n) {
        // Array of length n uses O(n) space
        const nums = new Array(n);
        // A list of length n occupies O(n) space
        const nodes = [];
        for (let i = 0; i < n; i++) {
            nodes.push(new ListNode(i));
        }
        // A hash table of length n occupies O(n) space
        const map = new Map();
        for (let i = 0; i < n; i++) {
            map.set(i, i.toString());
        }
    }
    ```

=== "TS"

    ```typescript title="space_complexity.ts"
    /* Linear order */
    function linear(n: number): void {
        // Array of length n uses O(n) space
        const nums = new Array(n);
        // A list of length n occupies O(n) space
        const nodes: ListNode[] = [];
        for (let i = 0; i < n; i++) {
            nodes.push(new ListNode(i));
        }
        // A hash table of length n occupies O(n) space
        const map = new Map();
        for (let i = 0; i < n; i++) {
            map.set(i, i.toString());
        }
    }
    ```

=== "Dart"

    ```dart title="space_complexity.dart"
    /* Linear order */
    void linear(int n) {
      // Array of length n uses O(n) space
      List<int> nums = List.filled(n, 0);
      // A list of length n occupies O(n) space
      List<ListNode> nodes = [];
      for (var i = 0; i < n; i++) {
        nodes.add(ListNode(i));
      }
      // A hash table of length n occupies O(n) space
      Map<int, String> map = HashMap();
      for (var i = 0; i < n; i++) {
        map.putIfAbsent(i, () => i.toString());
      }
    }
    ```

=== "Rust"

    ```rust title="space_complexity.rs"
    /* Linear order */
    #[allow(unused)]
    fn linear(n: i32) {
        // Array of length n uses O(n) space
        let mut nums = vec![0; n as usize];
        // A list of length n occupies O(n) space
        let mut nodes = Vec::new();
        for i in 0..n {
            nodes.push(ListNode::new(i))
        }
        // A hash table of length n occupies O(n) space
        let mut map = HashMap::new();
        for i in 0..n {
            map.insert(i, i.to_string());
        }
    }
    ```

=== "C"

    ```c title="space_complexity.c"
    /* Hash table */
    typedef struct {
        int key;
        int val;
        UT_hash_handle hh; // Implemented using uthash.h
    } HashTable;

    /* Linear order */
    void linear(int n) {
        // Array of length n uses O(n) space
        int *nums = malloc(sizeof(int) * n);
        free(nums);

        // A list of length n occupies O(n) space
        ListNode **nodes = malloc(sizeof(ListNode *) * n);
        for (int i = 0; i < n; i++) {
            nodes[i] = newListNode(i);
        }
        // Memory release
        for (int i = 0; i < n; i++) {
            free(nodes[i]);
        }
        free(nodes);

        // A hash table of length n occupies O(n) space
        HashTable *h = NULL;
        for (int i = 0; i < n; i++) {
            HashTable *tmp = malloc(sizeof(HashTable));
            tmp->key = i;
            tmp->val = i;
            HASH_ADD_INT(h, key, tmp);
        }

        // Memory release
        HashTable *curr, *tmp;
        HASH_ITER(hh, h, curr, tmp) {
            HASH_DEL(h, curr);
            free(curr);
        }
    }
    ```

=== "Kotlin"

    ```kotlin title="space_complexity.kt"
    /* Linear order */
    fun linear(n: Int) {
        // Array of length n uses O(n) space
        val nums = Array(n) { 0 }
        // A list of length n occupies O(n) space
        val nodes = mutableListOf<ListNode>()
        for (i in 0..<n) {
            nodes.add(ListNode(i))
        }
        // A hash table of length n occupies O(n) space
        val map = mutableMapOf<Int, String>()
        for (i in 0..<n) {
            map[i] = i.toString()
        }
    }
    ```

=== "Ruby"

    ```ruby title="space_complexity.rb"
    ### Linear time ###
    def linear(n)
      # A list of length n occupies O(n) space
      nums = Array.new(n, 0)

      # A hash table of length n occupies O(n) space
      hmap = {}
      for i in 0...n
        hmap[i] = i.to_s
      end
    end
    ```

As shown in the following figure, the recursion depth of this function is $n$, meaning that there are $n$ unreturned `linear_recur()` functions existing simultaneously, using $O(n)$ stack frame space:

=== "Python"

    ```python title="space_complexity.py"
    def linear_recur(n: int):
        """Linear order (recursive implementation)"""
        print("Recursion n =", n)
        if n == 1:
            return
        linear_recur(n - 1)
    ```

=== "C++"

    ```cpp title="space_complexity.cpp"
    /* Linear order (recursive implementation) */
    void linearRecur(int n) {
        cout << "Recursion n = " << n << endl;
        if (n == 1)
            return;
        linearRecur(n - 1);
    }
    ```

=== "Java"

    ```java title="space_complexity.java"
    /* Linear order (recursive implementation) */
    void linearRecur(int n) {
        System.out.println("Recursion n = " + n);
        if (n == 1)
            return;
        linearRecur(n - 1);
    }
    ```

=== "C#"

    ```csharp title="space_complexity.cs"
    /* Linear order (recursive implementation) */
    void LinearRecur(int n) {
        Console.WriteLine("Recursion n = " + n);
        if (n == 1) return;
        LinearRecur(n - 1);
    }
    ```

=== "Go"

    ```go title="space_complexity.go"
    /* Linear order (recursive implementation) */
    func spaceLinearRecur(n int) {
        fmt.Println("Recursion n =", n)
        if n == 1 {
            return
        }
        spaceLinearRecur(n - 1)
    }
    ```

=== "Swift"

    ```swift title="space_complexity.swift"
    /* Linear order (recursive implementation) */
    func linearRecur(n: Int) {
        print("Recursion n = \(n)")
        if n == 1 {
            return
        }
        linearRecur(n: n - 1)
    }
    ```

=== "JS"

    ```javascript title="space_complexity.js"
    /* Linear order (recursive implementation) */
    function linearRecur(n) {
        console.log(`Recursion n = ${n}`);
        if (n === 1) return;
        linearRecur(n - 1);
    }
    ```

=== "TS"

    ```typescript title="space_complexity.ts"
    /* Linear order (recursive implementation) */
    function linearRecur(n: number): void {
        console.log(`Recursion n = ${n}`);
        if (n === 1) return;
        linearRecur(n - 1);
    }
    ```

=== "Dart"

    ```dart title="space_complexity.dart"
    /* Linear order (recursive implementation) */
    void linearRecur(int n) {
      print('Recursion n = $n');
      if (n == 1) return;
      linearRecur(n - 1);
    }
    ```

=== "Rust"

    ```rust title="space_complexity.rs"
    /* Linear order (recursive implementation) */
    fn linear_recur(n: i32) {
        println!("Recursion n = {}", n);
        if n == 1 {
            return;
        };
        linear_recur(n - 1);
    }
    ```

=== "C"

    ```c title="space_complexity.c"
    /* Linear order (recursive implementation) */
    void linearRecur(int n) {
        printf("Recursion n = %d\r\n", n);
        if (n == 1)
            return;
        linearRecur(n - 1);
    }
    ```

=== "Kotlin"

    ```kotlin title="space_complexity.kt"
    /* Linear order (recursive implementation) */
    fun linearRecur(n: Int) {
        println("Recursion n = $n")
        if (n == 1)
            return
        linearRecur(n - 1)
    }
    ```

=== "Ruby"

    ```ruby title="space_complexity.rb"
    ### Linear space (recursive) ###
    def linear_recur(n)
      puts "Recursion n = #{n}"
      return if n == 1
      linear_recur(n - 1)
    end
    ```

![Linear order space complexity generated by recursive function](space_complexity.assets/space_complexity_recursive_linear.png){ class="animation-figure" }

<p align="center"> Figure 2-17 &nbsp; Linear order space complexity generated by recursive function </p>

### 3. &nbsp; Quadratic Order $O(n^2)$ {data-toc-label="3. &nbsp; Quadratic Order"}

Quadratic order is common in matrices and graphs, where the number of elements is quadratically related to $n$:

=== "Python"

    ```python title="space_complexity.py"
    def quadratic(n: int):
        """Quadratic order"""
        # A 2D list occupies O(n^2) space
        num_matrix = [[0] * n for _ in range(n)]
    ```

=== "C++"

    ```cpp title="space_complexity.cpp"
    /* Exponential order */
    void quadratic(int n) {
        // 2D list uses O(n^2) space
        vector<vector<int>> numMatrix;
        for (int i = 0; i < n; i++) {
            vector<int> tmp;
            for (int j = 0; j < n; j++) {
                tmp.push_back(0);
            }
            numMatrix.push_back(tmp);
        }
    }
    ```

=== "Java"

    ```java title="space_complexity.java"
    /* Exponential order */
    void quadratic(int n) {
        // Matrix uses O(n^2) space
        int[][] numMatrix = new int[n][n];
        // 2D list uses O(n^2) space
        List<List<Integer>> numList = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            List<Integer> tmp = new ArrayList<>();
            for (int j = 0; j < n; j++) {
                tmp.add(0);
            }
            numList.add(tmp);
        }
    }
    ```

=== "C#"

    ```csharp title="space_complexity.cs"
    /* Exponential order */
    void Quadratic(int n) {
        // Matrix uses O(n^2) space
        int[,] numMatrix = new int[n, n];
        // 2D list uses O(n^2) space
        List<List<int>> numList = [];
        for (int i = 0; i < n; i++) {
            List<int> tmp = [];
            for (int j = 0; j < n; j++) {
                tmp.Add(0);
            }
            numList.Add(tmp);
        }
    }
    ```

=== "Go"

    ```go title="space_complexity.go"
    /* Exponential order */
    func spaceQuadratic(n int) {
        // Matrix uses O(n^2) space
        numMatrix := make([][]int, n)
        for i := 0; i < n; i++ {
            numMatrix[i] = make([]int, n)
        }
    }
    ```

=== "Swift"

    ```swift title="space_complexity.swift"
    /* Exponential order */
    func quadratic(n: Int) {
        // 2D list uses O(n^2) space
        let numList = Array(repeating: Array(repeating: 0, count: n), count: n)
    }
    ```

=== "JS"

    ```javascript title="space_complexity.js"
    /* Exponential order */
    function quadratic(n) {
        // Matrix uses O(n^2) space
        const numMatrix = Array(n)
            .fill(null)
            .map(() => Array(n).fill(null));
        // 2D list uses O(n^2) space
        const numList = [];
        for (let i = 0; i < n; i++) {
            const tmp = [];
            for (let j = 0; j < n; j++) {
                tmp.push(0);
            }
            numList.push(tmp);
        }
    }
    ```

=== "TS"

    ```typescript title="space_complexity.ts"
    /* Exponential order */
    function quadratic(n: number): void {
        // Matrix uses O(n^2) space
        const numMatrix = Array(n)
            .fill(null)
            .map(() => Array(n).fill(null));
        // 2D list uses O(n^2) space
        const numList = [];
        for (let i = 0; i < n; i++) {
            const tmp = [];
            for (let j = 0; j < n; j++) {
                tmp.push(0);
            }
            numList.push(tmp);
        }
    }
    ```

=== "Dart"

    ```dart title="space_complexity.dart"
    /* Exponential order */
    void quadratic(int n) {
      // Matrix uses O(n^2) space
      List<List<int>> numMatrix = List.generate(n, (_) => List.filled(n, 0));
      // 2D list uses O(n^2) space
      List<List<int>> numList = [];
      for (var i = 0; i < n; i++) {
        List<int> tmp = [];
        for (int j = 0; j < n; j++) {
          tmp.add(0);
        }
        numList.add(tmp);
      }
    }
    ```

=== "Rust"

    ```rust title="space_complexity.rs"
    /* Exponential order */
    #[allow(unused)]
    fn quadratic(n: i32) {
        // Matrix uses O(n^2) space
        let num_matrix = vec![vec![0; n as usize]; n as usize];
        // 2D list uses O(n^2) space
        let mut num_list = Vec::new();
        for i in 0..n {
            let mut tmp = Vec::new();
            for j in 0..n {
                tmp.push(0);
            }
            num_list.push(tmp);
        }
    }
    ```

=== "C"

    ```c title="space_complexity.c"
    /* Exponential order */
    void quadratic(int n) {
        // 2D list uses O(n^2) space
        int **numMatrix = malloc(sizeof(int *) * n);
        for (int i = 0; i < n; i++) {
            int *tmp = malloc(sizeof(int) * n);
            for (int j = 0; j < n; j++) {
                tmp[j] = 0;
            }
            numMatrix[i] = tmp;
        }

        // Memory release
        for (int i = 0; i < n; i++) {
            free(numMatrix[i]);
        }
        free(numMatrix);
    }
    ```

=== "Kotlin"

    ```kotlin title="space_complexity.kt"
    /* Exponential order */
    fun quadratic(n: Int) {
        // Matrix uses O(n^2) space
        val numMatrix = arrayOfNulls<Array<Int>?>(n)
        // 2D list uses O(n^2) space
        val numList = mutableListOf<MutableList<Int>>()
        for (i in 0..<n) {
            val tmp = mutableListOf<Int>()
            for (j in 0..<n) {
                tmp.add(0)
            }
            numList.add(tmp)
        }
    }
    ```

=== "Ruby"

    ```ruby title="space_complexity.rb"
    ### Quadratic time ###
    def quadratic(n)
      # 2D list uses O(n^2) space
      Array.new(n) { Array.new(n, 0) }
    end
    ```

As shown in the following figure, the recursion depth of this function is $n$, and an array is initialized in each recursive function with lengths of $n$, $n-1$, $\dots$, $2$, $1$, with an average length of $n / 2$, thus occupying $O(n^2)$ space overall:

=== "Python"

    ```python title="space_complexity.py"
    def quadratic_recur(n: int) -> int:
        """Quadratic order (recursive implementation)"""
        if n <= 0:
            return 0
        # Array nums length is n, n-1, ..., 2, 1
        nums = [0] * n
        return quadratic_recur(n - 1)
    ```

=== "C++"

    ```cpp title="space_complexity.cpp"
    /* Quadratic order (recursive implementation) */
    int quadraticRecur(int n) {
        if (n <= 0)
            return 0;
        vector<int> nums(n);
        cout << "In recursion n = " << n << ", nums length = " << nums.size() << endl;
        return quadraticRecur(n - 1);
    }
    ```

=== "Java"

    ```java title="space_complexity.java"
    /* Quadratic order (recursive implementation) */
    int quadraticRecur(int n) {
        if (n <= 0)
            return 0;
        // Array nums has length n, n-1, ..., 2, 1
        int[] nums = new int[n];
        System.out.println("In recursion n = " + n + ", nums length = " + nums.length);
        return quadraticRecur(n - 1);
    }
    ```

=== "C#"

    ```csharp title="space_complexity.cs"
    /* Quadratic order (recursive implementation) */
    int QuadraticRecur(int n) {
        if (n <= 0) return 0;
        int[] nums = new int[n];
        Console.WriteLine("Recursion n = " + n + ", nums length = " + nums.Length);
        return QuadraticRecur(n - 1);
    }
    ```

=== "Go"

    ```go title="space_complexity.go"
    /* Quadratic order (recursive implementation) */
    func spaceQuadraticRecur(n int) int {
        if n <= 0 {
            return 0
        }
        nums := make([]int, n)
        fmt.Printf("In recursion n = %d, nums length = %d \n", n, len(nums))
        return spaceQuadraticRecur(n - 1)
    }
    ```

=== "Swift"

    ```swift title="space_complexity.swift"
    /* Quadratic order (recursive implementation) */
    @discardableResult
    func quadraticRecur(n: Int) -> Int {
        if n <= 0 {
            return 0
        }
        // Array nums has length n, n-1, ..., 2, 1
        let nums = Array(repeating: 0, count: n)
        print("In recursion n = \(n), nums length = \(nums.count)")
        return quadraticRecur(n: n - 1)
    }
    ```

=== "JS"

    ```javascript title="space_complexity.js"
    /* Quadratic order (recursive implementation) */
    function quadraticRecur(n) {
        if (n <= 0) return 0;
        const nums = new Array(n);
        console.log(`In recursion n = ${n}, nums length = ${nums.length}`);
        return quadraticRecur(n - 1);
    }
    ```

=== "TS"

    ```typescript title="space_complexity.ts"
    /* Quadratic order (recursive implementation) */
    function quadraticRecur(n: number): number {
        if (n <= 0) return 0;
        const nums = new Array(n);
        console.log(`In recursion n = ${n}, nums length = ${nums.length}`);
        return quadraticRecur(n - 1);
    }
    ```

=== "Dart"

    ```dart title="space_complexity.dart"
    /* Quadratic order (recursive implementation) */
    int quadraticRecur(int n) {
      if (n <= 0) return 0;
      List<int> nums = List.filled(n, 0);
      print('In recursion n = $n, nums length = ${nums.length}');
      return quadraticRecur(n - 1);
    }
    ```

=== "Rust"

    ```rust title="space_complexity.rs"
    /* Quadratic order (recursive implementation) */
    fn quadratic_recur(n: i32) -> i32 {
        if n <= 0 {
            return 0;
        };
        // Array nums has length n, n-1, ..., 2, 1
        let nums = vec![0; n as usize];
        println!("In recursion n = {}, nums length = {}", n, nums.len());
        return quadratic_recur(n - 1);
    }
    ```

=== "C"

    ```c title="space_complexity.c"
    /* Quadratic order (recursive implementation) */
    int quadraticRecur(int n) {
        if (n <= 0)
            return 0;
        int *nums = malloc(sizeof(int) * n);
        printf("In recursion n = %d, nums length = %d\r\n", n, n);
        int res = quadraticRecur(n - 1);
        free(nums);
        return res;
    }
    ```

=== "Kotlin"

    ```kotlin title="space_complexity.kt"
    /* Quadratic order (recursive implementation) */
    tailrec fun quadraticRecur(n: Int): Int {
        if (n <= 0)
            return 0
        // Array nums has length n, n-1, ..., 2, 1
        val nums = Array(n) { 0 }
        println("In recursion n = $n, nums length = ${nums.size}")
        return quadraticRecur(n - 1)
    }
    ```

=== "Ruby"

    ```ruby title="space_complexity.rb"
    ### Quadratic space (recursive) ###
    def quadratic_recur(n)
      return 0 unless n > 0

      # Array nums has length n, n-1, ..., 2, 1
      nums = Array.new(n, 0)
      quadratic_recur(n - 1)
    end
    ```

![Quadratic order space complexity generated by recursive function](space_complexity.assets/space_complexity_recursive_quadratic.png){ class="animation-figure" }

<p align="center"> Figure 2-18 &nbsp; Quadratic order space complexity generated by recursive function </p>

### 4. &nbsp; Exponential Order $O(2^n)$ {data-toc-label="4. &nbsp; Exponential Order"}

Exponential order is common in binary trees. Observe the following figure: a "full binary tree" with $n$ levels has $2^n - 1$ nodes, occupying $O(2^n)$ space:

=== "Python"

    ```python title="space_complexity.py"
    def build_tree(n: int) -> TreeNode | None:
        """Exponential order (build full binary tree)"""
        if n == 0:
            return None
        root = TreeNode(0)
        root.left = build_tree(n - 1)
        root.right = build_tree(n - 1)
        return root
    ```

=== "C++"

    ```cpp title="space_complexity.cpp"
    /* Driver Code */
    TreeNode *buildTree(int n) {
        if (n == 0)
            return nullptr;
        TreeNode *root = new TreeNode(0);
        root->left = buildTree(n - 1);
        root->right = buildTree(n - 1);
        return root;
    }
    ```

=== "Java"

    ```java title="space_complexity.java"
    /* Driver Code */
    TreeNode buildTree(int n) {
        if (n == 0)
            return null;
        TreeNode root = new TreeNode(0);
        root.left = buildTree(n - 1);
        root.right = buildTree(n - 1);
        return root;
    }
    ```

=== "C#"

    ```csharp title="space_complexity.cs"
    /* Driver Code */
    TreeNode? BuildTree(int n) {
        if (n == 0) return null;
        TreeNode root = new(0) {
            left = BuildTree(n - 1),
            right = BuildTree(n - 1)
        };
        return root;
    }
    ```

=== "Go"

    ```go title="space_complexity.go"
    /* Driver Code */
    func buildTree(n int) *TreeNode {
        if n == 0 {
            return nil
        }
        root := NewTreeNode(0)
        root.Left = buildTree(n - 1)
        root.Right = buildTree(n - 1)
        return root
    }
    ```

=== "Swift"

    ```swift title="space_complexity.swift"
    /* Driver Code */
    func buildTree(n: Int) -> TreeNode? {
        if n == 0 {
            return nil
        }
        let root = TreeNode(x: 0)
        root.left = buildTree(n: n - 1)
        root.right = buildTree(n: n - 1)
        return root
    }
    ```

=== "JS"

    ```javascript title="space_complexity.js"
    /* Driver Code */
    function buildTree(n) {
        if (n === 0) return null;
        const root = new TreeNode(0);
        root.left = buildTree(n - 1);
        root.right = buildTree(n - 1);
        return root;
    }
    ```

=== "TS"

    ```typescript title="space_complexity.ts"
    /* Driver Code */
    function buildTree(n: number): TreeNode | null {
        if (n === 0) return null;
        const root = new TreeNode(0);
        root.left = buildTree(n - 1);
        root.right = buildTree(n - 1);
        return root;
    }
    ```

=== "Dart"

    ```dart title="space_complexity.dart"
    /* Driver Code */
    TreeNode? buildTree(int n) {
      if (n == 0) return null;
      TreeNode root = TreeNode(0);
      root.left = buildTree(n - 1);
      root.right = buildTree(n - 1);
      return root;
    }
    ```

=== "Rust"

    ```rust title="space_complexity.rs"
    /* Driver Code */
    fn build_tree(n: i32) -> Option<Rc<RefCell<TreeNode>>> {
        if n == 0 {
            return None;
        };
        let root = TreeNode::new(0);
        root.borrow_mut().left = build_tree(n - 1);
        root.borrow_mut().right = build_tree(n - 1);
        return Some(root);
    }
    ```

=== "C"

    ```c title="space_complexity.c"
    /* Driver Code */
    TreeNode *buildTree(int n) {
        if (n == 0)
            return NULL;
        TreeNode *root = newTreeNode(0);
        root->left = buildTree(n - 1);
        root->right = buildTree(n - 1);
        return root;
    }
    ```

=== "Kotlin"

    ```kotlin title="space_complexity.kt"
    /* Driver Code */
    fun buildTree(n: Int): TreeNode? {
        if (n == 0)
            return null
        val root = TreeNode(0)
        root.left = buildTree(n - 1)
        root.right = buildTree(n - 1)
        return root
    }
    ```

=== "Ruby"

    ```ruby title="space_complexity.rb"
    ### Exponential space (build full binary tree) ###
    def build_tree(n)
      return if n == 0

      TreeNode.new.tap do |root|
        root.left = build_tree(n - 1)
        root.right = build_tree(n - 1)
      end
    end
    ```

![Exponential order space complexity generated by full binary tree](space_complexity.assets/space_complexity_exponential.png){ class="animation-figure" }

<p align="center"> Figure 2-19 &nbsp; Exponential order space complexity generated by full binary tree </p>

### 5. &nbsp; Logarithmic Order $O(\log n)$ {data-toc-label="5. &nbsp; Logarithmic Order"}

Logarithmic order is common in divide-and-conquer algorithms. For example, merge sort: given an input array of length $n$, each recursion divides the array in half from the midpoint, forming a recursion tree of height $\log n$, using $O(\log n)$ stack frame space.

Another example is converting a number to a string. Given a positive integer $n$, it has $\lfloor \log_{10} n \rfloor + 1$ digits, i.e., the corresponding string length is $\lfloor \log_{10} n \rfloor + 1$, so the space complexity is $O(\log_{10} n + 1) = O(\log n)$.

## 2.4.4 &nbsp; Trading Time for Space

Ideally, we hope that both the time complexity and space complexity of an algorithm can reach optimal. However, in practice, optimizing both time complexity and space complexity simultaneously is usually very difficult.

**Reducing time complexity usually comes at the cost of increasing space complexity, and vice versa**. Sacrificing memory space to improve execution speed is called "trading space for time"; the reverse is called "trading time for space".

The choice of which approach depends on which aspect we value more. In most cases, time is more precious than space, so "trading space for time" is usually the more common strategy. Of course, when the data volume is very large, controlling space complexity is also very important.