A linked list is given such that each node contains an additional random pointer which could point to any node in the list or null.

Return a deep copy of the list.

思路：第一遍正常复制链表，同时用哈希表保存链表中原始节点和新节点的对应关系，第二遍遍历链表的时候，再复制随机域。

这是一种典型的空间换时间的做法，n个节点，需要大小为O(n)的哈希表，同时时间复杂度可以降低到O(n)。

/**
 * 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 head;
		}
		HashMap<RandomListNode, RandomListNode> map = new HashMap<RandomListNode, RandomListNode>();
		RandomListNode res = null;
		RandomListNode taiListNode = null;
		RandomListNode cur = head;
		while (cur != null) {
			if (res == null) {
				res = new RandomListNode(cur.label);
				res.next = res.random = null;
				taiListNode = res;
				map.put(head, res);
			}
			else{
				taiListNode.next = new RandomListNode(cur.label);
				taiListNode = taiListNode.next;
				map.put(cur, taiListNode);
				
			}
			cur = cur.next;
		}
		taiListNode.next = null;
		cur = head;
		taiListNode = res;
		while (cur != null) {
			taiListNode.random = (RandomListNode)map.get((RandomListNode)cur.random);
			cur = cur.next;
			taiListNode = taiListNode.next;
		}
		return res;
    }
}