101. Symmetric Tree

Description

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.

Solution

/**
 * 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) {
    "use strict";
    if (!root) {
        return true;
    }
    function check(node1, node2) {
        if (!node1 || !node2) {
            return node1 === node2;
        }
        if (node1.val === node2.val) {
            if (check(node1.left, node2.right) &&
                check(node1.right, node2.left)) {
                return true;
            }
        }
        return false;
    }
    return check(root.left, root.right);
};