141 Linked List Cycle(判断链表是否有环Medium)

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     bool hasCycle(ListNode *head) {
12         if(head==NULL)return false;
13         ListNode *p1,*p2;
14         p1=p2=head;
15         while(p2->next&&p2->next->next){      //要判断p->next->next为空先要判断p->next是否为空，以免产生p->next->next不存在的蛋疼问题
16             p1=p1->next;
17             p2=p2->next->next;
18             if(p1==p2)
19                 return true;
20         }
21         return false;
22     }
23 };