Copy List with Random Pointer [LeetCode]

A linked list is given such that each node contains an additional random pointer which could point to any node in the list or null.

Return a deep copy of the list.

Solution: Uses map to recorde node mapping between old linked list and new linked list.

    RandomListNode *copyRandomList(RandomListNode *head) {
        map<RandomListNode *, RandomListNode *> old_to_new;
        RandomListNode * current = head;
        RandomListNode * new_head = NULL;
        RandomListNode * pre = NULL;
        while(current != NULL) {
            RandomListNode * node = new RandomListNode(current->label);
            old_to_new[current] = node;    
            if(new_head == NULL){
                new_head = node;
                pre = node;
            }else if(pre != NULL){
                pre->next = node;
                pre = pre->next;
            }
            
            current = current->next;
        }
        
        current = head;
        RandomListNode * new_current = new_head;
        while(current != NULL && new_current != NULL) {
            if(current->random != NULL)
                new_current->random = old_to_new[current->random];   
            else
                new_current->random = NULL; 
            current = current->next;
            new_current = new_current->next;
        }
        return new_head;
    }