/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
        ListNode*p1=head;
        ListNode*p2=head;
        while(p2){
            //p1向前走一步
            p1=p1->next;
            //p2向前走两部
            p2=p2->next;
            if(p2)p2=p2->next;
            //推断p2是否追上了p1
            if(p2 && p2==p1)return true;
        }
        return false;
    }
};