LeetCode-Flatten Binary Tree to Linked List

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void flatten(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        flat(root);
    }
    
    TreeNode *flat(TreeNode *root)
    {
        if (root == NULL || (root->left == NULL && root->right == NULL))
            return root;
        TreeNode *rightMost = flat(root->left);
        TreeNode *oldRight = root->right;
        if (rightMost != NULL)
        {
            root->right = root->left;
            root->left = NULL;
            rightMost->right = oldRight;    
        }
        return oldRight != NULL ? flat(oldRight) : rightMost;
    }
};