LeetCode 138 Copy List with Random Pointer

class Solution {
public:
    RandomListNode* ans;
    map<int,RandomListNode*> m;
    RandomListNode *copyRandomList(RandomListNode *head) {
        
       if(head==NULL)
           return ans;
       
        dfs(ans,head);
        return ans;
    }
    
    void dfs(RandomListNode* &ans,RandomListNode *head)
    {
        if(head==NULL) return;
        if(m[head->label]==NULL)
        {
             m[head->label] = new RandomListNode(head->label);
        }
        ans = new RandomListNode(head->label);
        if(head->random==NULL)
            ans->random = NULL;
        else
        {
            if(m[head->random->label]==NULL)
                m[head->random->label] = new RandomListNode(head->random->label);
            ans->random = m[head->random->label];
        }

        dfs(ans->next,head->next);
    }
};