环形链表II 142 使用快慢指针（C++实现）

 /**
  * Definition for singly-linked list.
  * struct ListNode {
  *     int val;
  *     ListNode *next;
  *     ListNode(int x) : val(x), next(NULL) {}
  * };
  */
 class Solution {
 public:
     ListNode *detectCycle(ListNode *head) {
         ListNode* slow=head;
         ListNode* fast=head;
         ListNode* meet=NULL;

         while(fast)
         {
             slow=slow->next;
             fast=fast->next;
             if(!fast)
             return NULL;
             fast=fast->next;
             if(slow==fast)
                 meet=fast;
             if(meet==head)
                 return meet;
             else if(meet)
             {
             meet=meet->next;
             head=head->next;
             }
         }
         return NULL;
     }
 };