[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;   
    }
};