算法复习：双指针（对撞指针、快慢指针）

class Solution {
public:
    ListNode *detectCycle(ListNode *head)
    {
        int lable=0;
        struct ListNode *slow,*fast,*pp;
        if(head==NULL)
            return NULL;
        if(head->next==NULL)
            return NULL;
        slow=head->next;
        if(slow->next==NULL)
            return NULL;
        fast=slow->next;
        while(slow!=fast)// 步骤一：使用快慢指针判断链表是否有环
        {
            if(fast->next==NULL)
            {
                lable=1;
                break;
            }
            fast=fast->next;//快指针走两步
            if(fast->next==NULL)
            {
                lable=1;
                break;
            }
            fast=fast->next;
            slow=slow->next;//慢指针走一步
        }
        if(lable==1)
            return NULL;
        pp=head;
        while(pp!=slow)// 步骤二：若有环，找到入环开始的节点
        {
            pp=pp->next;
            slow=slow->next;
        }
        return pp;
    }
};