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*/

/**
 * 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) {
    return root == null || isSymmetricHelper(root.left,root.right)
};

var isSymmetricHelper = function(left,right){
    if(left == null || right == null)
        return left == right
    if(left.val != right.val)
        return false
    return isSymmetricHelper(left.left,right.right) && isSymmetricHelper(left.right,right.left)
};