LeetCode 138. Copy List with Random Pointer

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.

这个题目想了好久，觉得应该用HashMap 之类的，可是就是不知道怎么用。最后还是参考了论坛上的答案。

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

O(N)的时间复杂度， O(N)的空间复杂度。