如果有两个头结点指针，一个走的快，一个走的慢，那么若干步以后，快的指针总会超过慢的指针一圈。

bool IsCycleList(ListNode* pHead){	if(pHead== NULL) 		return false;	if (pHead->m_pNext == NULL)	{		return false;	}	ListNode* pFastNode   = pHead;	ListNode* pSlowNode  = pHead;	//如果为偶数个，pFastNode->m_pNext->m_pNext为空；如果为奇数个，pFast->m_pNext为空；	while(pFastNode->m_pNext != NULL && pFastNode->m_pNext->m_pNext != NULL)	{		pFastNode = pFastNode->m_pNext->m_pNext;		pSlowNode = pSlowNode->m_pNext;		if (pSlowNode == pFastNode)		{			break;		}	}	if (pFastNode == NULL || pFastNode->m_pNext == NULL)	{		return false;	}	else	{		return true;	}}

完整代码测试：

// CycleList.cpp : 定义控制台应用程序的入口点。//#include "stdafx.h"#include <iostream>using namespace  std;struct ListNode {	int       m_nValue;	ListNode* m_pNext;	ListNode(int k):m_nValue(k),m_pNext(NULL)	{	}};bool IsCycleList(ListNode* pHead){	if(pHead== NULL) 		return false;	if (pHead->m_pNext == NULL)	{		return false;	}	ListNode* pFastNode   = pHead;	ListNode* pSlowNode  = pHead;	//如果为偶数个，pFastNode->m_pNext->m_pNext为空；如果为奇数个，pFast->m_pNext为空；	while(pFastNode->m_pNext != NULL && pFastNode->m_pNext->m_pNext != NULL)	{		pFastNode = pFastNode->m_pNext->m_pNext;		pSlowNode = pSlowNode->m_pNext;		if (pSlowNode == pFastNode)		{			break;		}	}	if (pFastNode == NULL || pFastNode->m_pNext == NULL)	{		return false;	}	else	{		return true;	}}int _tmain(int argc, _TCHAR* argv[]){	ListNode* head  = new ListNode(1);	ListNode* Node1 = new ListNode(2);	ListNode* Node2 = new ListNode(3);	ListNode* Node3 = new ListNode(4);	ListNode* Node4 = new ListNode(5);	ListNode* Node5 = new ListNode(6);	ListNode* Node6 = new ListNode(7);	head->m_pNext = Node1;	Node1->m_pNext = Node2;	Node2->m_pNext = Node3;	Node3->m_pNext = Node4;	Node4->m_pNext = Node5;	Node5->m_pNext = Node6;	Node6->m_pNext = head;	bool b = IsCycleList(head);	cout<<b<<endl;	getchar();	return 0;}

C++ 算法之 是否为环形链表