static boolean isMirror(TreeNode left, TreeNode right) {
        if (left == null && right == null) return true;
        else if (right == null) {
            return false;
        } else if (left == null) {
            return false;
        } else {
            if (left.val != right.val) return false;

            return isMirror(left.right, right.left) && isMirror(left.left, right.right);
        }
    }

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