添加（二叉树的迭代遍历.md）：增加typescript版本

Author: xiaofei-2020

problems/二叉树的迭代遍历.md

@@ -454,6 +454,61 @@ var postorderTraversal = function(root, res = []) {
 };
 ```
 
+TypeScript:
+
+```typescript
+// 前序遍历（迭代法）
+function preorderTraversal(root: TreeNode | null): number[] {
+    if (root === null) return [];
+    let res: number[] = [];
+    let helperStack: TreeNode[] = [];
+    let curNode: TreeNode = root;
+    helperStack.push(curNode);
+    while (helperStack.length > 0) {
+        curNode = helperStack.pop()!;
+        res.push(curNode.val);
+        if (curNode.right !== null) helperStack.push(curNode.right);
+        if (curNode.left !== null) helperStack.push(curNode.left);
+    }
+    return res;
+};
+
+// 中序遍历（迭代法）
+function inorderTraversal(root: TreeNode | null): number[] {
+    let helperStack: TreeNode[] = [];
+    let res: number[] = [];
+    if (root === null) return res;
+    let curNode: TreeNode | null = root;
+    while (curNode !== null || helperStack.length > 0) {
+        if (curNode !== null) {
+            helperStack.push(curNode);
+            curNode = curNode.left;
+        } else {
+            curNode = helperStack.pop()!;
+            res.push(curNode.val);
+            curNode = curNode.right;
+        }
+    }
+    return res;
+};
+
+// 后序遍历（迭代法）
+function postorderTraversal(root: TreeNode | null): number[] {
+    let helperStack: TreeNode[] = [];
+    let res: number[] = [];
+    let curNode: TreeNode;
+    if (root === null) return res;
+    helperStack.push(root);
+    while (helperStack.length > 0) {
+        curNode = helperStack.pop()!;
+        res.push(curNode.val);
+        if (curNode.left !== null) helperStack.push(curNode.left);
+        if (curNode.right !== null) helperStack.push(curNode.right);
+    }
+    return res.reverse();
+};
+```
+
 Swift:
 
 > 迭代法前序遍历