添加 二叉树的迭代遍历 Swift版本

Author: qxuewei

problems/二叉树的迭代遍历.md

@@ -454,6 +454,79 @@ var postorderTraversal = function(root, res = []) {
 };
 ```
 
+Swift:
+
+> 迭代法前序遍历
+```swift
+func preorderTraversal(_ root: TreeNode?) -> [Int] {
+    var res = [Int]()
+    if root == nil {
+        return res
+    }
+    var stack = [TreeNode]()
+    stack.append(root!)
+    while !stack.isEmpty {
+        let node = stack.popLast()!
+        res.append(node.val)
+        if node.right != nil {
+            stack.append(node.right!)
+        }
+        if node.left != nil {
+            stack.append(node.left!)
+        }
+    }
+    return res
+}
+```
+
+> 迭代法中序遍历
+```swift
+func inorderTraversal(_ root: TreeNode?) -> [Int] {
+    var res = [Int]()
+    if root == nil {
+        return res
+    }
+    var stack = [TreeNode]()
+    var cur: TreeNode? = root
+    while cur != nil || !stack.isEmpty {
+        if cur != nil {
+            stack.append(cur!)
+            cur = cur!.left
+        } else {
+            cur = stack.popLast()
+            res.append(cur!.val)
+            cur = cur!.right
+        }
+    }
+    return res
+}
+```
+
+> 迭代法后序遍历
+```swift
+func postorderTraversal(_ root: TreeNode?) -> [Int] {
+    var res = [Int]()
+    if root == nil {
+        return res
+    }
+    var stack = [TreeNode]()
+    stack.append(root!)
+    // res 存储 中 -> 右 -> 左
+    while !stack.isEmpty {
+        let node = stack.popLast()!
+        res.append(node.val)
+        if node.left != nil  {
+            stack.append(node.left!)
+        }
+        if node.right != nil  {
+            stack.append(node.right!)
+        }
+    }
+    // res 翻转
+    res.reverse()
+    return res
+}
+```
 
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