[leetcode]Binary Tree Upside Down

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* rotate(TreeNode* root) {
        if (root == nullptr || root->left == nullptr) {
            ans = root;
            return root;
        }
        TreeNode* tree = rotate(root->left);
        tree->right = root;
        tree->left = root->right;
        root->left = nullptr;
        root->right = nullptr;
        return tree->right;
    }
    
    TreeNode *upsideDownBinaryTree(TreeNode *root) {
        ans = nullptr;
        rotate(root);
        return ans;
    }
private:
    TreeNode* ans;
};