链表中环的入口结点

题目描述

给一个链表，若其中包含环，请找出该链表的环的入口结点，否则，输出null。

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
        val(x), next(NULL) {
    }
};
*/
class Solution {
public:
    ListNode* EntryNodeOfLoop(ListNode* pHead)
    {
    	ListNode* slow = pHead;
    	ListNode* fast = pHead;

    	int step = 2;
    	while(fast&&step>0)
    	{
    		fast = fast->next;
    		step--;
    	}

    	bool cycle = false;
    	while(slow&&fast)
    	{
    		if(slow == fast)
    		{
    			cycle = true;
    			break;
    		}

    		slow = slow->next;
    		fast = fast->next;
    		if(fast)
    			fast = fast->next;
    	}

    	if(!cycle)
    		return NULL;

    	int lenth=1;
    	fast = fast->next;
    	while(fast != slow)
    	{
    		lenth++;
    		fast = fast->next;
    	}
        
        ListNode* move = pHead;
        for(int i = lenth;i>0;i--)
        {
            move = move->next;
        }
        while(move != pHead)
        {
            move = move->next;
            pHead = pHead->next;
        }
        return move;

    }
};