添加 二叉树的递归遍历 Swift版本

Author: qxuewei

problems/二叉树的递归遍历.md

@@ -409,5 +409,56 @@ int* postorderTraversal(struct TreeNode* root, int* returnSize){
 }
 ```
 
+Swift:
+前序遍历：（144.二叉树的前序遍历）
+```Swift
+func preorderTraversal(_ root: TreeNode?) -> [Int] {
+    var res = [Int]()
+    preorder(root, res: &res)
+    return res
+}
+func preorder(_ root: TreeNode?, res: inout [Int]) {
+    if root == nil {
+        return
+    }
+    res.append(root!.val)
+    preorder(root!.left, res: &res)
+    preorder(root!.right, res: &res)
+}
+```
+
+中序遍历：（94. 二叉树的中序遍历）
+```Swift
+func inorderTraversal(_ root: TreeNode?) -> [Int] {
+    var res = [Int]()
+    inorder(root, res: &res)
+    return res
+}
+func inorder(_ root: TreeNode?, res: inout [Int]) {
+    if root == nil {
+        return
+    }
+    inorder(root!.left, res: &res)
+    res.append(root!.val)
+    inorder(root!.right, res: &res)
+}
+```
+
+后序遍历：（145. 二叉树的后序遍历）
+```Swift
+func postorderTraversal(_ root: TreeNode?) -> [Int] {
+    var res = [Int]()
+    postorder(root, res: &res)
+    return res
+}
+func postorder(_ root: TreeNode?, res: inout [Int]) {
+    if root == nil {
+        return
+    }
+    postorder(root!.left, res: &res)
+    postorder(root!.right, res: &res)
+    res.append(root!.val)
+}
+```
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