feat: add Swift codes for binary_tree_traversal article

codes/swift/chapter_tree/binary_tree_dfs.swift

@@ -0,0 +1,70 @@
+/**
+ * File: binary_tree_dfs.swift
+ * Created Time: 2023-01-18
+ * Author: nuomi1 (nuomi1@qq.com)
+ */
+
+import utils
+
+// 初始化列表，用于存储遍历序列
+var list: [Int] = []
+
+/* 前序遍历 */
+func preOrder(root: TreeNode?) {
+    guard let root = root else {
+        return
+    }
+    // 访问优先级：根结点 -> 左子树 -> 右子树
+    list.append(root.val)
+    preOrder(root: root.left)
+    preOrder(root: root.right)
+}
+
+/* 中序遍历 */
+func inOrder(root: TreeNode?) {
+    guard let root = root else {
+        return
+    }
+    // 访问优先级：左子树 -> 根结点 -> 右子树
+    inOrder(root: root.left)
+    list.append(root.val)
+    inOrder(root: root.right)
+}
+
+/* 后序遍历 */
+func postOrder(root: TreeNode?) {
+    guard let root = root else {
+        return
+    }
+    // 访问优先级：左子树 -> 右子树 -> 根结点
+    postOrder(root: root.left)
+    postOrder(root: root.right)
+    list.append(root.val)
+}
+
+@main
+enum BinaryTreeDFS {
+    /* Driver Code */
+    static func main() {
+        /* 初始化二叉树 */
+        // 这里借助了一个从数组直接生成二叉树的函数
+        let root = TreeNode.listToTree(list: [1, 2, 3, 4, 5, 6, 7])!
+        print("\n初始化二叉树\n")
+        PrintUtil.printTree(root: root)
+
+        /* 前序遍历 */
+        list.removeAll()
+        preOrder(root: root)
+        print("\n前序遍历的结点打印序列 = \(list)")
+
+        /* 中序遍历 */
+        list.removeAll()
+        inOrder(root: root)
+        print("\n中序遍历的结点打印序列 = \(list)")
+
+        /* 后序遍历 */
+        list.removeAll()
+        postOrder(root: root)
+        print("\n后序遍历的结点打印序列 = \(list)")
+    }
+}