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.

这题用map做其实比较简单，但是一开始没想明白越想越乱，最后看了下别人的实现，思路还是很清晰的，代码如下所示：

1 /** 2  * Definition for singly-linked list with a random pointer. 3  * struct RandomListNode { 4  *     int label; 5  *     RandomListNode *next, *random; 6  *     RandomListNode(int x) : label(x), next(NULL), random(NULL) {} 7  * }; 8  */ 9 10 class Solution {11 public:12     RandomListNode *copyRandomList(RandomListNode *head) {13         unordered_map
     
       m;14         RandomListNode helper1(0), * p1 = &helper1, helper2(0), *p2 = &helper2;15         p1->next = head;16         while(p1->next){17             p1 = p1->next;18             RandomListNode * tmpNode = new RandomListNode(p1->label);19             m[p1] = tmpNode;20             p2 = p2->next = tmpNode;21         }22         p1 = &helper1;23         while(p1->next){24             p1 = p1->next;25             if(p1->random){26                 m[p1]->random = m[p1->random];27             }28         }29         return helper2.next;30     }31 };