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;
     }