Symmetric Tree

Given a binary tree, check whether it is a mirror of itself(ie, symmetric around its center)

For example, this tree is symmetric:

     1
    / \
   2   2
  / \ / \
 3  4 4  3

But the following tree is not.

    1
   / \
  2   2
   \   \
   3    3

public boolean isSymmetric(TreeNode root) {
    if (root == null) {
        return true;
    }
    return helper(root.left, root.right);
}

private boolean helper(TreeNode left, TreeNode right) {
    if (left == null && right == null) {
        return true;
    }
    if (left == null || right == null) {
        return false;
    }
    return (left.val == right.val) && helper(left.left, right.right) 
        && helper(left.right, right.left);
}