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