每日一题(11) - 统计有环单链表的环中结点个数

/*思路：在快慢指针相遇位置肯定是在环中，此时一个指针不动，让另一个指针转圈，再次相遇时经过的结点个数就是环中结点个数*/
int GetCountOfCycle(ListNode* pHead)
{
	assert(pHead != NULL);
	ListNode* pFast = pHead;
	ListNode* pSlow = pHead;
	int nCount = 1;//第一个结点是相遇点

	while (pFast->m_pNext && pFast->m_pNext->m_pNext)
	{
		pFast = pFast->m_pNext->m_pNext;
		pSlow = pSlow->m_pNext;
		if (pFast == pSlow)//碰撞点
		{
			//计算环中结点个数
			pFast = pFast->m_pNext;
			while (pFast != pSlow)
			{
				nCount++;
				pFast = pFast->m_pNext;
			}
			return nCount;
		}
	}
	return 0;
}

#include <iostream>
#include <assert.h>
using namespace std;

struct ListNode
{
	int m_Data;
	ListNode* m_pNext;
};

bool IsCycle(ListNode* pHead)
{
	assert(pHead != NULL);
	ListNode* pFast = pHead;
	ListNode* pSlow = pHead;
	while (pFast->m_pNext && pFast->m_pNext->m_pNext)
	{
		pFast = pFast->m_pNext->m_pNext;
		pSlow = pSlow->m_pNext;
		if (pFast == pSlow)
		{
			return true;
		}
	}
	return false;
}

/*思路：在快慢指针相遇位置肯定是在环中，此时一个指针不动，让另一个指针转圈，再次相遇时经过的结点个数就是环中结点个数*/
int GetCountOfCycle(ListNode* pHead)
{
	assert(pHead != NULL);
	ListNode* pFast = pHead;
	ListNode* pSlow = pHead;
	int nCount = 1;//第一个结点是相遇点

	while (pFast->m_pNext && pFast->m_pNext->m_pNext)
	{
		pFast = pFast->m_pNext->m_pNext;
		pSlow = pSlow->m_pNext;
		if (pFast == pSlow)//碰撞点
		{
			//计算环中结点个数
			pFast = pFast->m_pNext;
			while (pFast != pSlow)
			{
				nCount++;
				pFast = pFast->m_pNext;
			}
			return nCount;
		}
	}
	return 0;
}
void CreateList(ListNode** pHead,int nLen)//头指针使用指针的指针
{
	assert(*pHead == NULL && nLen > 0);
	ListNode* pCur = NULL;
	ListNode* pNewNode = NULL;
	for (int i = 0;i < nLen;i++)
	{
		pNewNode = new ListNode;
		cin>>pNewNode->m_Data;
		pNewNode->m_pNext = NULL;

		if (*pHead == NULL)
		{
			*pHead = pNewNode;
			pCur = *pHead;
		}
		else
		{
			pCur->m_pNext = pNewNode;
			pCur = pNewNode;
		}
	}
}

/*把链表最后一个结点指向第三个结点*/
void CreateCycle(ListNode* pHead)
{
	assert(pHead);
	ListNode* pLastNode = pHead;
	while (pLastNode->m_pNext)
	{
		pLastNode = pLastNode->m_pNext;
	}
	if (pHead->m_pNext && pHead->m_pNext->m_pNext)
	{
		pLastNode->m_pNext =  pHead->m_pNext->m_pNext;
	}
}

int main()
{
	int nLen = 0;
	bool bIsCircle = false;
	ListNode* pHead = NULL;
	ListNode* pHeadSec = NULL;
	ListNode* pEntry = NULL;

	cout<<"please input node num: ";
	cin >> nLen;
	CreateList(&pHead,nLen);//注意传参使用引用
	CreateCycle(pHead);
	bIsCircle = IsCycle(pHead);
	if (bIsCircle)
	{
		cout<<"有环！"<<endl;
		cout<<"环中结点个数: "<<GetCountOfCycle(pHead)<<endl;
	}
	else
	{
		cout<<"无环"<<endl;
	}

	cout<<"please input node num: ";
	cin >> nLen;
	CreateList(&pHeadSec,nLen);//注意传参使用引用
	CreateCycle(pHeadSec);
	bIsCircle = IsCycle(pHeadSec);
	if (bIsCircle)
	{
		cout<<"有环！"<<endl;
		cout<<"环中结点个数: "<<GetCountOfCycle(pHeadSec)<<endl;
	}
	else
	{
		cout<<"无环"<<endl;
	}
	
	system("pause");
	return 1;
}