Copy List with Random Pointer (with HashMap and n recursive calls)

RecursiveVersion.java

/**
 * Definition for singly-linked list with a random pointer.
 * class RandomListNode {
 *     int label;
 *     RandomListNode next, random;
 *     RandomListNode(int x) { this.label = x; }
 * };
 */
public class RecursiveVersion {
    public RandomListNode copyRandomList(RandomListNode head) {
        HashMap<RandomListNode,RandomListNode> nodes = new HashMap<RandomListNode,RandomListNode>();
        return copyListHelper(head, nodes);
    }
    public RandomListNode copyListHelper(RandomListNode head, HashMap<RandomListNode,RandomListNode> nodes)  {
        if(head==null)  return null;
        if(nodes.containsKey(head))  return nodes.get(head);
        RandomListNode ret = new RandomListNode(head.label);
        nodes.put(head,ret);
        ret.next = copyListHelper(head.next, nodes);
        ret.random = copyListHelper(head.random,nodes);
        return ret;
    }
}