/*

struct ListNode {

    int val;

    struct ListNode *next;

    ListNode(int x) :

        val(x), next(NULL) {

    }

};

*/

class Solution {

public:

    ListNode* EntryNodeOfLoop(ListNode* pHead)

    {

        ListNode* pFast = pHead;

		ListNode* pSlow = pHead;

		while ((pFast != nullptr) && (pFast->next != nullptr) && (pSlow != nullptr))

		{

			pFast = pFast->next->next;

			pSlow = pSlow->next;

			if (pFast == pSlow)

			{

				break;

			}

		}

		if ((pFast == nullptr) || (pFast->next == nullptr) || (pSlow == nullptr))

		{

			return nullptr;

		}

		pFast = pHead;

		while (pFast != pSlow)

		{

			pFast = pFast->next;

			pSlow = pSlow->next;

		}

		return pFast;

    }

};