请完成一个函数，输入一个二叉树，该函数输出它的镜像。

例如输入：

     4
   /   \
  2     7
 / \   / \
1   3 6   9

镜像输出：

     4
   /   \
  7     2
 / \   / \
9   6 3   1

示例 1：

输入：root = [4,2,7,1,3,6,9]
输出：[4,7,2,9,6,3,1]

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/er-cha-shu-de-jing-xiang-lcof
解答：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode mirrorTree(TreeNode root) {
        Stack<TreeNode> s1 = new Stack();
        s1.push(root);
        if (root == null){
            return null;
        }
        while(!s1.isEmpty()){
            TreeNode tree = s1.pop();
            TreeNode temp;
            temp = tree.left;
            tree.left = tree.right;
            tree.right = temp;
            if (tree.right != null) {
                s1.push(tree.right);
            }
            if (tree.left != null) {
                s1.push(tree.left);
            }
        }
        return root;
    }
}