copy list with random pointer

public RandomListNode (RandomListNode head) {
    if (head == null)
        return head;
    HashMap<RandomListNode, RandomListNode> map = new HashMap<>();
    RandomListNode ptr = head;
    map.put(ptr, new RandomListNode(ptr.label));
    while (ptr != null) {
        if(ptr.next != null) {
            map.put(ptr.next, new RandomListNode(ptr.next.label));
            map.get(ptr).next = map.get(ptr.next);
        }
        ptr = ptr.next;
    }
    ptr = head;
    while (ptr != null) {
        RandomListNode rand = ptr.random;
        if (rand != null)
            map.get(ptr).random = map.get(rand);
        ptr = ptr.next;
    }
    return map.get(head);
}