Construct String from Binary Tree

import { TreeNode } from '@algorithm/lib';
 
export function tree2str(root: TreeNode | null): string {
  if (root === null) {
    return '';
  }
  if (root.left === null && root.right === null) {
    return root.val.toString();
  }
  if (root.right === null) {
    return `${root.val}(${tree2str(root.left)})`;
  }
  return `${root.val}(${tree2str(root.left)})(${tree2str(root.right)})`;
}