Feature/chapter dynamic programming swift (#608)

* feat: add Swift codes for intro_to_dynamic_programming article

* feat: add Swift codes for dp_problem_features article

* feat: add Swift codes for dp_solution_pipeline article

* feat: add Swift codes for knapsack_problem article

* feat: add Swift codes for unbounded_knapsack_problem article

* feat: add Swift codes for edit_distance_problem article

codes/swift/chapter_dynamic_programming/climbing_stairs_dfs_mem.swift

@@ -0,0 +1,40 @@
+/**
+ * File: climbing_stairs_dfs_mem.swift
+ * Created Time: 2023-07-15
+ * Author: nuomi1 (nuomi1@qq.com)
+ */
+
+/* 记忆化搜索 */
+func dfs(i: Int, mem: inout [Int]) -> Int {
+    // 已知 dp[1] 和 dp[2] ，返回之
+    if i == 1 || i == 2 {
+        return i
+    }
+    // 若存在记录 dp[i] ，则直接返回之
+    if mem[i] != -1 {
+        return mem[i]
+    }
+    // dp[i] = dp[i-1] + dp[i-2]
+    let count = dfs(i: i - 1, mem: &mem) + dfs(i: i - 2, mem: &mem)
+    // 记录 dp[i]
+    mem[i] = count
+    return count
+}
+
+/* 爬楼梯：记忆化搜索 */
+func climbingStairsDFSMem(n: Int) -> Int {
+    // mem[i] 记录爬到第 i 阶的方案总数，-1 代表无记录
+    var mem = Array(repeating: -1, count: n + 1)
+    return dfs(i: n, mem: &mem)
+}
+
+@main
+enum ClimbingStairsDFSMem {
+    /* Driver Code */
+    static func main() {
+        let n = 9
+
+        let res = climbingStairsDFSMem(n: n)
+        print("爬 \(n) 阶楼梯共有 \(res) 种方案")
+    }
+}