Copy List with Random Pointer

public class Solution {
    public RandomListNode copyRandomList(RandomListNode head) {
        //http://www.cnblogs.com/springfor/p/3864457.html
        if(head==null) return null;
        
        //soln 1 HashMap 遍历2次list，所以时间复杂度是O(2n)=O(n)，然后使用了HashMap，所以空间复杂度是O(n)。
        
        RandomListNode newhd = new RandomListNode(head.label);
        RandomListNode oldhd = head;
        RandomListNode oldp = head.next;
        RandomListNode newp = newhd;
        HashMap<RandomListNode, RandomListNode> hm = new HashMap<RandomListNode, RandomListNode>();
        hm.put(oldhd, newhd); //Map<K,V> , which is key，which is value？
        
        while(oldp!=null){
            RandomListNode newnode = new RandomListNode(oldp.label);
            newp.next = newnode;
            newp = newp.next;
            hm.put(oldp,newp);
            oldp = oldp.next;
            
        }
        
        oldp = head;
  
  
        newp = newhd;
        
        while(oldp!=null){
            newp.random = hm.get(oldp.random);  
            newp = newp.next;
            oldp = oldp.next;
        }
        
        return newhd;
        
        
        //soln 2 a) replica old nodes twice, b) replica node.next(which is node's replica).random = node.random.next(which is random's replica) , c) separate node and its replica
        RandomListNode node = head;
        while(node!=null){
            RandomListNode newnode = new RandomListNode(node.label);
            newnode.next = node.next;
            node.next = newnode;   // a) replica old nodes twice
            node =newnode.next;
        }
        
        node = head;
        while(node!=null){
            if(node.random!=null)
                node.next.random =  node.random.next; // b) replica node.next(which is node's replica).random = node.random.next(which is random's replica)
            node = node.next.next;
        }
        
        node = head;
        RandomListNode newhd = head.next;
        while(node!=null){
            RandomListNode rep = node.next;
            node.next = rep.next;
            if(rep.next!=null)
                rep.next = rep.next.next;
            node = node.next;
        }
        
        return newhd;
    }
}