剑指offer: 链表中环的入口节点
class Solution {
public:
    ListNode* EntryNodeOfLoop(ListNode* pHead)
    {
        ListNode* fast=pHead,*low=pHead;
        while(fast&&fast->next)
        {
            fast=fast->next->next;
            low=low->next;
            if(fast==low)
                break;
        }
        if(!fast||!fast->next)
            return NULL;
        low=pHead; //low从链表的头出发
        while(fast!=low)
        {
            fast=fast->next;
            low=low->next;
        }
        return low;
    }
};