src/symmetric-tree/res.js

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {boolean}
 */
var isSymmetric = function(root) {
  const isMirror = (t1, t2) => {
    if (t1 == null && t2 == null) return true;
    if (t1 == null || t2 == null) return false;

    return t1.val === t2.val && isMirror(t1.left, t2.right) && isMirror(t1.right, t2.left);
  }
  
  return isMirror(root, root);
};