添加（二叉树理论基础.md）：增加typescript版本

problems/二叉树理论基础.md

@@ -227,7 +227,23 @@ function TreeNode(val, left, right) {
 }
 ```
 
+TypeScript：
+
+```typescript
+class TreeNode {
+    public val: number;
+    public left: TreeNode | null;
+    public right: TreeNode | null;
+    constructor(val?: number, left?: TreeNode, right?: TreeNode) {
+        this.val = val === undefined ? 0 : val;
+        this.left = left === undefined ? null : left;
+        this.right = right === undefined ? null : right;
+    }
+}
+```
+
 Swift:
+
 ```Swift
 class TreeNode<T> {
     var value: T