//解题思路：
    //左右子节点互换，然后递归就ok了
    public TreeNode Mirror (TreeNode pRoot) {
        if(pRoot == null) {
            return null;
        }
        TreeNode temp = pRoot.left;
        pRoot.left = pRoot.right;
        pRoot.right = temp;
        Mirror(pRoot.left);
        Mirror(pRoot.right);
        return pRoot;
    }