symmetric-tree.js

// Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).
//
//
// For example, this binary tree [1,2,2,3,4,4,3] is symmetric:
//
//     1
//    / \
//   2   2
//  / \ / \
// 3  4 4  3
//
//
//
// But the following [1,2,2,null,3,null,3]  is not:
//
//     1
//    / \
//   2   2
//    \   \
//    3    3
//
//
//
//
// Note:
// Bonus points if you could solve it both recursively and iteratively.
//


/**
 * 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) {
  if (!root) return true
  var _isSymmetric = function(l, r) {
    if (!l || !r) {
      return l == r
    }
    if (l.val !== r.val) return false
    return _isSymmetric(l.left, r.right) && _isSymmetric(l.right, r.left)
  }

  return _isSymmetric(root.left, root.right)
};