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;     } };