1. /**
    2.  * Definition for a binary tree node.
    3.  * function TreeNode(val, left, right) {
    4.  *     this.val = (val===undefined ? 0 : val)
    5.  *     this.left = (left===undefined ? null : left)
    6.  *     this.right = (right===undefined ? null : right)
    7.  * }
    8.  */
    9. /**
    10.  * @param {TreeNode} root
    11.  * @return {boolean}
    12.  */
    13. var isSymmetric = function(root) {
    14.   const check = (left, right) => {
    15.     if(!left && !right) return true
    16.     if(!left || !right) return false
    18.     return left.val === right.val && check(left.left, right.right) && check(left.right, right.left)
    19.   }
    21.   return check(root.left, root.right)
    22. };