添加（二叉树的递归遍历.md）：增加typescript版本

problems/二叉树的递归遍历.md

@@ -358,7 +358,51 @@ var postorderTraversal = function(root) {
 };
 ```
 
+TypeScript:
+
+```typescript
+// 前序遍历
+function preorderTraversal(node: TreeNode | null): number[] {
+    function traverse(node: TreeNode | null, res: number[]): void {
+        if (node === null) return;
+        res.push(node.val);
+        traverse(node.left, res);
+        traverse(node.right, res);
+    }
+    const res: number[] = [];
+    traverse(node, res);
+    return res;
+}
+
+// 中序遍历
+function inorderTraversal(node: TreeNode | null): number[] {
+    function traverse(node: TreeNode | null, res: number[]): void {
+        if (node === null) return;
+        traverse(node.left, res);
+        res.push(node.val);
+        traverse(node.right, res);
+    }
+    const res: number[] = [];
+    traverse(node, res);
+    return res;
+}
+
+// 后序遍历
+function postorderTraversal(node: TreeNode | null): number[] {
+    function traverse(node: TreeNode | null, res: number[]): void {
+        if (node === null) return;
+        traverse(node.left, res);
+        traverse(node.right, res);
+        res.push(node.val);
+    }
+    const res: number[] = [];
+    traverse(node, res);
+    return res;
+}
+```
+
 C:
+
 ```c
 //前序遍历：
 void preOrderTraversal(struct TreeNode* root, int* ret, int* returnSize) {