text: /**
   * Definition for a binary tree node.
   * function TreeNode(val, left, right) {
   *     this.val = (val===undefined ? 0 : val)
   *     this.left = (left===undefined ? null : left)
   *     this.right = (right===undefined ? null : right)
   * }
   */
  /**
   * @param {TreeNode} root
   * @return {number[]}
   */
  var postorderTraversal = function (root) {
    if (!root) return [];
    return [
      ...postorderTraversal(root.left),
      ...postorderTraversal(root.right),
      root.val,
    ];
  };
  