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.

 1     RandomListNode *copyRandomList(RandomListNode *head) {
 2         map<RandomListNode *, RandomListNode *> old_to_new;
 3         RandomListNode * current = head;
 4         RandomListNode * new_head = NULL;
 5         RandomListNode * pre = NULL;
 6         while(current != NULL) {
 7             RandomListNode * node = new RandomListNode(current->label);
 8             old_to_new[current] = node;    
 9             if(new_head == NULL){
10                 new_head = node;
11                 pre = node;
12             }else if(pre != NULL){
13                 pre->next = node;
14                 pre = pre->next;
15             }
16             
17             current = current->next;
18         }
19         
20         current = head;
21         RandomListNode * new_current = new_head;
22         while(current != NULL && new_current != NULL) {
23             if(current->random != NULL)
24                 new_current->random = old_to_new[current->random];   
25             else
26                 new_current->random = NULL; 
27             current = current->next;
28             new_current = new_current->next;
29         }
30         return new_head;
31     }