LeetCode 题解之Linked List Cycle II

1、题目描述

2、问题分析

使用快慢指针方法判断链表是否有环，然后寻找环开始的节点。

3、代码

 ListNode *detectCycle(ListNode *head) {
         if( head == NULL || head->next == NULL ){
             return NULL ;
         }

         ListNode* fast = head;
         ListNode* slow = head;

         while( fast != NULL && slow != NULL ){

             if( fast->next !=  NULL ){
                 fast = fast->next->next;
             }else{
                 return NULL ;
             }
             slow = slow->next;
             if( fast == slow ){
                 break;
             }
         }

         if( fast == NULL || slow == NULL ){
             return NULL;
         }

         ListNode* start = head;
         while( start != slow ){
             start = start->next ;
             slow = slow->next;
         }

         return start;

     }