Merge pull request #1161 from xiaofei-2020/tree34

添加（0538.把二叉搜索树转换为累加树.md）：增加typescript版本

problems/0538.把二叉搜索树转换为累加树.md

@@ -312,5 +312,47 @@ struct TreeNode* convertBST(struct TreeNode* root){
 }
 ```
 
+## TypeScript
+
+> 递归法
+
+```typescript
+function convertBST(root: TreeNode | null): TreeNode | null {
+    let pre: number = 0;
+    function recur(root: TreeNode | null): void {
+        if (root === null) return;
+        recur(root.right);
+        root.val += pre;
+        pre = root.val;
+        recur(root.left);
+    }
+    recur(root);
+    return root;
+};
+```
+
+> 迭代法
+
+```typescript
+function convertBST(root: TreeNode | null): TreeNode | null {
+    const helperStack: TreeNode[] = [];
+    let curNode: TreeNode | null = root;
+    let pre: number = 0;
+    while (curNode !== null || helperStack.length > 0) {
+        while (curNode !== null) {
+            helperStack.push(curNode);
+            curNode = curNode.right;
+        }
+        curNode = helperStack.pop()!;
+        curNode.val += pre;
+        pre = curNode.val;
+        curNode = curNode.left;
+    }
+    return root;
+};
+```
+
+
+
 -----------------------
 <div align="center"><img src=https://code-thinking.cdn.bcebos.com/pics/01二维码一.jpg width=500> </img></div>