Merge pull request #1063 from xiaofei-2020/tree4

添加（二叉树的统一迭代法.md）：增加typescript版本

problems/二叉树的统一迭代法.md

@@ -522,7 +522,75 @@ var postorderTraversal = function(root, res = []) {
 
 ```
 
+TypeScript：
 
+```typescript
+// 前序遍历（迭代法）
+function preorderTraversal(root: TreeNode | null): number[] {
+    let helperStack: (TreeNode | null)[] = [];
+    let res: number[] = [];
+    let curNode: TreeNode | null;
+    if (root === null) return res;
+    helperStack.push(root);
+    while (helperStack.length > 0) {
+        curNode = helperStack.pop()!;
+        if (curNode !== null) {
+            if (curNode.right !== null) helperStack.push(curNode.right);
+            helperStack.push(curNode);
+            helperStack.push(null);
+            if (curNode.left !== null) helperStack.push(curNode.left);
+        } else {
+            curNode = helperStack.pop()!;
+            res.push(curNode.val);
+        }
+    }
+    return res;
+};
+
+// 中序遍历（迭代法）
+function inorderTraversal(root: TreeNode | null): number[] {
+    let helperStack: (TreeNode | null)[] = [];
+    let res: number[] = [];
+    let curNode: TreeNode | null;
+    if (root === null) return res;
+    helperStack.push(root);
+    while (helperStack.length > 0) {
+        curNode = helperStack.pop()!;
+        if (curNode !== null) {
+            if (curNode.right !== null) helperStack.push(curNode.right);
+            helperStack.push(curNode);
+            helperStack.push(null);
+            if (curNode.left !== null) helperStack.push(curNode.left);
+        } else {
+            curNode = helperStack.pop()!;
+            res.push(curNode.val);
+        }
+    }
+    return res;
+};
+
+// 后序遍历（迭代法）
+function postorderTraversal(root: TreeNode | null): number[] {
+    let helperStack: (TreeNode | null)[] = [];
+    let res: number[] = [];
+    let curNode: TreeNode | null;
+    if (root === null) return res;
+    helperStack.push(root);
+    while (helperStack.length > 0) {
+        curNode = helperStack.pop()!;
+        if (curNode !== null) {
+            if (curNode.right !== null) helperStack.push(curNode.right);
+            helperStack.push(curNode);
+            helperStack.push(null);
+            if (curNode.left !== null) helperStack.push(curNode.left);
+        } else {
+            curNode = helperStack.pop()!;
+            res.push(curNode.val);
+        }
+    }
+    return res;
+};
+```
 
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 <div align="center"><img src=https://code-thinking.cdn.bcebos.com/pics/01二维码一.jpg width=500> </img></div>