【每日一题-3】链表带环问题

判断链表是否带环？如果带环，环的长度是多少？环的入口节点是什么？

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

//判断链表是否带环？
ListNode* IsLoop(ListNode* pHead)
{
	if (NULL == pHead)
	{
		return NULL;
	}
	ListNode* pfast = pHead;
	ListNode* pslow = pHead;
	while (pHead->next != NULL && pHead->next->next != NULL)
	{
		pfast = pfast->next->next;
		pslow = pslow->next;
		if (pfast == pslow)
		{
			return pslow;
		}
	}
	return NULL;
}

//若带环求环的长度？说明已经存在环
int GetLoopListLen(ListNode* pMeetNode)
{
	if (pMeetNode == NULL)		//pMeetNode为快慢指针相遇点
		return 0;
	int count = 0;
	ListNode* cur = pMeetNode;
	while (cur != pMeetNode)
	{
		++count;
		cur = cur->next;
	}
	return count;
}

//若带环求环的入口点？
ListNode* EntryNodeOfLoop(ListNode* pHead, ListNode* pMeet)
{
	if (NULL == pHead && NULL == pMeet)
	{
		return NULL;
	}
	ListNode* pFirst = pHead;
	ListNode* pSecond = pMeet;
	while (pFirst != pMeet)
	{
		pFirst = pFirst->next;
		pSecond = pSecond->next;
	}
	return pFirst;
}

class A
{
public:
	static A GetA()
	{
		A a;
		return a;
	}

	static void deleteA(A* a)
	{
		delete a;
		a = NULL;
	}
private:
	A()
	{
		cout << "A" << endl;
	}

	~A()
	{
		cout << "~A" << endl;
	}
};

class StackType
{
public:
	static StackType* CreateObject()
	{
		return new StackType;
	}
private:
	StackType()
	{}
	int _p;
};

class HeapType
{
public:
	static HeapType* CreateObject()
	{
		return new HeapType;
	}
private:
	HeapType()
	{}
	int _p;
};