Problem Copy List with Random Pointer

 1     public RandomListNode copyRandomList(RandomListNode head) {
 2        Map<RandomListNode, RandomListNode> map = new HashMap<RandomListNode, RandomListNode>();
 3 
 4         if (head == null) return null;
 5 
 6         RandomListNode q = head;
 7         RandomListNode p = null;
 8         while (q != null ) {
 9             p = new RandomListNode(q.label);
10             map.put(q, p);
11             q = q.next;
12         } 
13 
14         q = head;
15         RandomListNode root = map.get(head);
16         while (q != null) {
17             p = map.get(q);
18             p.next = map.get(q.next);
19             p.random = map.get(q.random);
20             q = q.next;
21         }
22 
23         return root;
24     }