[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;
    }
};