141. Linked List Cycle

Given a linked list, determine if it has a cycle in it.

   

解题思路：

#include<windows.h>

#include<iostream>

using namespace std;

struct ListNode {

    int val;

    ListNode *next;

    ListNode(int x) : val(x), next(NULL) {}

};

    

class Solution {

public:

    bool hasCycle(ListNode *head) {

        if(head==NULL||head->next==NULL)return false;

        ListNode*p=head;

        ListNode*q=head->next->next;

        while(p!=NULL&&q!=NULL){

            if(p==q)return true;

            p=p->next;

            if(q->next!=NULL)q=q->next->next;

            else

                break;

        }

        return false;

    }

};