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public RandomListNode copyRandomList(RandomListNode head) {
  if (head == null) return null;
  
  Map<RandomListNode, RandomListNode> map = new HashMap<RandomListNode, RandomListNode>();
  
  // loop 1. copy all the nodes
  RandomListNode node = head;
  while (node != null) {
    map.put(node, new RandomListNode(node.label));
    node = node.next;
  }
  
  // loop 2. assign next and random pointers
  node = head;
  while (node != null) {
    map.get(node).next = map.get(node.next);
    map.get(node).random = map.get(node.random);
    node = node.next;
  }
  
  return map.get(head);
}

/**
 * Definition for singly-linked list with a random pointer.
 * class RandomListNode {
 *     int label;
 *     RandomListNode next, random;
 *     RandomListNode(int x) { this.label = x; }
 * };
 */
public class Solution {
    public RandomListNode copyRandomList(RandomListNode head) {
        if (head == null) return null;
        RandomListNode iter = head, next;
        while (iter != null) {
            next = iter.next;
            RandomListNode copy = new RandomListNode(iter.label);
            iter.next = copy;
            copy.next = next;
            iter = next;
        }
        
        iter = head;
        while (iter != null) {
            if (iter.random != null) {
                iter.next.random = iter.random.next;
            }
            iter = iter.next.next;
        }
        
        iter = head;
        RandomListNode newHead = new RandomListNode(-1);
        RandomListNode copyIter = newHead;
        
        while (iter != null) {
            next = iter.next.next;
        
            copyIter.next = iter.next;
            copyIter = copyIter.next;
            
            iter.next = next;
            iter = next;
        }
        return newHead.next;
    }
}