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]限制：
0 <= 节点个数 <= 1000
//leetcode submit region begin(Prohibit modification and deletion)
/**
* 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) {
doFlip(root);
return root;
}
private void doFlip(TreeNode node) {
if (node == null) {
return;
}
TreeNode left = node.left;
node.left = node.right;
node.right = left;
doFlip(node.left);
doFlip(node.right);
}
}
//leetcode submit region end(Prohibit modification and deletion)
