feat: add Swift codes for backtracking_algorithm article (#480)

* fix: compile error

* fix: package define

* feat: add Swift codes for backtracking_algorithm article

codes/swift/chapter_backtracking/preorder_traversal_i_compact.swift

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+/**
+ * File: preorder_traversal_i_compact.swift
+ * Created Time: 2023-04-30
+ * Author: nuomi1 (nuomi1@qq.com)
+ */
+
+import utils
+
+var res: [TreeNode] = []
+
+/* 前序遍历：例题一 */
+func preOrder(root: TreeNode?) {
+    guard let root = root else {
+        return
+    }
+    if root.val == 7 {
+        // 记录解
+        res.append(root)
+    }
+    preOrder(root: root.left)
+    preOrder(root: root.right)
+}
+
+@main
+enum PreorderTraversalICompact {
+    /* Driver Code */
+    static func main() {
+        let root = TreeNode.listToTree(list: [1, 7, 3, 4, 5, 6, 7])
+        print("\n初始化二叉树")
+        PrintUtil.printTree(root: root)
+
+        // 前序遍历
+        res = []
+        preOrder(root: root)
+
+        print("\n输出所有值为 7 的节点")
+        var vals: [Int] = []
+        for node in res {
+            vals.append(node.val)
+        }
+        print(vals)
+    }
+}

codes/swift/chapter_backtracking/preorder_traversal_ii_compact.swift

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+/**
+ * File: preorder_traversal_ii_compact.swift
+ * Created Time: 2023-04-30
+ * Author: nuomi1 (nuomi1@qq.com)
+ */
+
+import utils
+
+var path: [TreeNode] = []
+var res: [[TreeNode]] = []
+
+/* 前序遍历：例题二 */
+func preOrder(root: TreeNode?) {
+    guard let root = root else {
+        return
+    }
+    // 尝试
+    path.append(root)
+    if root.val == 7 {
+        // 记录解
+        res.append(path)
+    }
+    preOrder(root: root.left)
+    preOrder(root: root.right)
+    // 回退
+    path.removeLast()
+}
+
+@main
+enum PreorderTraversalIICompact {
+    /* Driver Code */
+    static func main() {
+        let root = TreeNode.listToTree(list: [1, 7, 3, 4, 5, 6, 7])
+        print("\n初始化二叉树")
+        PrintUtil.printTree(root: root)
+
+        // 前序遍历
+        path = []
+        res = []
+        preOrder(root: root)
+
+        print("\n输出所有根节点到节点 7 的路径")
+        for path in res {
+            var vals: [Int] = []
+            for node in path {
+                vals.append(node.val)
+            }
+            print(vals)
+        }
+    }
+}

codes/swift/chapter_backtracking/preorder_traversal_iii_compact.swift

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+/**
+ * File: preorder_traversal_iii_compact.swift
+ * Created Time: 2023-04-30
+ * Author: nuomi1 (nuomi1@qq.com)
+ */
+
+import utils
+
+var path: [TreeNode] = []
+var res: [[TreeNode]] = []
+
+/* 前序遍历：例题三 */
+func preOrder(root: TreeNode?) {
+    // 剪枝
+    guard let root = root, root.val != 3 else {
+        return
+    }
+    // 尝试
+    path.append(root)
+    if root.val == 7 {
+        // 记录解
+        res.append(path)
+    }
+    preOrder(root: root.left)
+    preOrder(root: root.right)
+    // 回退
+    path.removeLast()
+}
+
+@main
+enum PreorderTraversalIIICompact {
+    /* Driver Code */
+    static func main() {
+        let root = TreeNode.listToTree(list: [1, 7, 3, 4, 5, 6, 7])
+        print("\n初始化二叉树")
+        PrintUtil.printTree(root: root)
+
+        // 前序遍历
+        path = []
+        res = []
+        preOrder(root: root)
+
+        print("\n输出所有根节点到节点 7 的路径，且路径中不包含值为 3 的节点")
+        for path in res {
+            var vals: [Int] = []
+            for node in path {
+                vals.append(node.val)
+            }
+            print(vals)
+        }
+    }
+}

codes/swift/chapter_backtracking/preorder_traversal_iii_template.swift

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+/**
+ * File: preorder_traversal_iii_template.swift
+ * Created Time: 2023-04-30
+ * Author: nuomi1 (nuomi1@qq.com)
+ */
+
+import utils
+
+/* 判断当前状态是否为解 */
+func isSolution(state: [TreeNode]) -> Bool {
+    !state.isEmpty && state.last!.val == 7
+}
+
+/* 记录解 */
+func recordSolution(state: [TreeNode], res: inout [[TreeNode]]) {
+    res.append(state)
+}
+
+/* 判断在当前状态下，该选择是否合法 */
+func isValid(state: [TreeNode], choice: TreeNode?) -> Bool {
+    choice != nil && choice!.val != 3
+}
+
+/* 更新状态 */
+func makeChoice(state: inout [TreeNode], choice: TreeNode) {
+    state.append(choice)
+}
+
+/* 恢复状态 */
+func undoChoice(state: inout [TreeNode], choice: TreeNode) {
+    state.removeLast()
+}
+
+/* 回溯算法：例题三 */
+func backtrack(state: inout [TreeNode], choices: [TreeNode], res: inout [[TreeNode]]) {
+    // 检查是否为解
+    if isSolution(state: state) {
+        recordSolution(state: state, res: &res)
+        return
+    }
+    // 遍历所有选择
+    for choice in choices {
+        // 剪枝：检查选择是否合法
+        if isValid(state: state, choice: choice) {
+            // 尝试：做出选择，更新状态
+            makeChoice(state: &state, choice: choice)
+            // 进行下一轮选择
+            backtrack(state: &state, choices: [choice.left, choice.right].compactMap { $0 }, res: &res)
+            // 回退：撤销选择，恢复到之前的状态
+            undoChoice(state: &state, choice: choice)
+        }
+    }
+}
+
+@main
+enum preorder_traversal_iii_template {
+    /* Driver Code */
+    static func main() {
+        let root = TreeNode.listToTree(list: [1, 7, 3, 4, 5, 6, 7])
+        print("\n初始化二叉树")
+        PrintUtil.printTree(root: root)
+
+        // 回溯算法
+        var state: [TreeNode] = []
+        var res: [[TreeNode]] = []
+        backtrack(state: &state, choices: [root].compactMap { $0 }, res: &res)
+
+        print("\n输出所有根节点到节点 7 的路径，且路径中不包含值为 3 的节点")
+        for path in res {
+            var vals: [Int] = []
+            for node in path {
+                vals.append(node.val)
+            }
+            print(vals)
+        }
+    }
+}