剑指 Offer 27：二叉树的镜像

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

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

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

class Solution {
public:
    TreeNode* mirrorTree(TreeNode* root) {
        if(!root) return root;
        root->left=mirrorTree(root->left);
        root->right=mirrorTree(root->right);
        swap(root->left,root->right);
        return root;
    }
};