Symmetric Tree

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

For example, this binary tree is symmetric: 

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

But the following is not:

    1
   / \
  2   2
   \   \
   3    3

public class Solution {
    //Time: O(n)
    public boolean isSymmetric(TreeNode root) {
        if (root == null) {
            return true;
        }
        return check(root.left, root.right);
    }
    
    private boolean check(TreeNode left, TreeNode right) {
        if (left == null && right == null) {
            return true;
        }
        if (left == null || right == null) {
            return false;
        }
        if (left.val != right.val) {
            return false;
        }
        return check(left.left, right.right) && check(left.right, right.left);
    }
}