[LeetCode] Copy List with Random Pointe

题目的关键是要让新链表和原有链表发送关联，可以通过这种关联来设置新链表的random pointer

思路：将新链表的元素插入到原有链表元素的后面，如下图所示，就可以根据原有链表的radom->next 就是新链表的random指针

所以分3步骤：

1 新元素插入

2 设置新链表的random

3 拆分大链表，回复old link 和new link

 /**
  * Definition for singly-linked list with a random pointer.
  * struct RandomListNode {
  *     int label;
  *     RandomListNode *next, *random;
  *     RandomListNode(int x) : label(x), next(NULL), random(NULL) {}
  * };
  */

 class Solution {
 public:
     RandomListNode *copyRandomList(RandomListNode *head) {

         RandomListNode* pOld = head;
         RandomListNode* pNew = NULL;
         RandomListNode* pRes = NULL;

         if(head == NULL) return NULL;

         // insert every new node after old new node
         while(pOld)
         {
             pNew = new RandomListNode(pOld->label);
             if(pOld == head) pRes = pNew;
             pNew->next = pOld->next;
             pOld->next = pNew;
             pOld = pNew->next;
         }

         pOld = head;
         // constrct new's random pointer
         while(pOld)
         {
             pNew = pOld->next;
             if(pOld->random == NULL)
                 pNew->random == NULL;
             else
                 pNew->random = pOld->random->next;
             pOld = pNew->next;
         }

         // recover old's and new's next pointer
         //恢复old list 和new list

         pOld = head;

         while(pOld)
         {
             pNew = pOld->next;
             if(pNew == NULL)
                 pOld->next = NULL;
             else
                 pOld->next = pNew->next;
             pOld = pNew;
         }

         return pRes;
     }
 };