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;
        
        
    }