[Leetcode] Binary Tree Upside Down

class Solution {private:    TreeNode *upside;public:    TreeNode *upsideDownBinaryTree(TreeNode *root) {        if (root)            helper(root);        return upside;    }    TreeNode* helper(TreeNode *root) {        if (root->left == NULL) {            upside = root;            return root;        }        TreeNode *parent = helper(root->left);        parent->right = root;        parent->left = root->right;        root->left = NULL;        root->right = NULL;        return root;    }};