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) {
        // 1
        /*
        if (root == NULL) return;
        if (root->left == NULL) {
            flatten(root->right);
            return;
        }
        
        TreeNode *lr = root->left;
        while (lr->right != NULL) lr = lr->right;
        lr->right = root->right;
        root->right = root->left;
        root->left = NULL;
        flatten(root->right);
        */
        
        // 2
        flattenHelper(root);
        
        return;
    }
    
private:
    TreeNode* flattenHelper(TreeNode *root) {
        if (root == NULL || (root->left == NULL && root->right == NULL)) return root;
        TreeNode *l = flattenHelper(root->left);
        TreeNode *r = root->right;
        if (l != NULL) {
            l->right = root->right;
            root->right = root->left;
            root->left = NULL;
        }
        
        r = flattenHelper(r);
        return (r == NULL ? l : r);
    }
};