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;
}
}
