#include<iostream>
#include"ListNode.h"


class Solution2{

    public:
        ListNode* findCycle(ListNode* list){
        
            ListNode* fast = list;
            ListNode* slow = list;
            ListNode* meet = NULL;
//查詢到快慢指標相遇位置並記錄
            while(fast){
                
                fast = fast->next;
                slow = slow->next;
                
                if(!fast){
                    return NULL;
                }

                fast = fast->next;

                if(fast == slow){
                    meet = fast;
                    break;
                }         
            }
//判斷相遇位置在末尾
           if(meet ==  NULL){
 
               return NULL;
           }

//一個指標從頭節點開始，一個指標從相遇節點開始遍歷，
//直到相遇，就是環開始的位置
             while(meet && list){

                 if(list == meet){
                     //返回相遇位置
                     return list;
                 }

                 list = list->next;
                 meet = meet->next;
             }

             return NULL;
            
        }
};
    



int main()
{
  ListNode a(8);
  ListNode b(1);
  ListNode c(2);
  ListNode d(3);
  ListNode e(4);
  a.next=&b;
  b.next=&c;
  c.next=&d;
  d.next=&c;
  std::cout<<"the List has been generated!"<<std::endl;
  ListNode* p = &a;
  Solution2 solution;
  ListNode*   result =solution.findCycle(p);
   std::cout<<"result: "<<result->val<<std::endl;
    return 0;
}