# LC 417. Linked List Cycle II

## 题目描述

Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.

Note: Do not modify the linked list.

Example 1:

Input: head = [3,2,0,-4], pos = 1
Output: tail connects to node index 1
Explanation: There is a cycle in the linked list, where tail connects to the second node.


Example 2:

Input: head = [1,2], pos = 0
Output: tail connects to node index 0
Explanation: There is a cycle in the linked list, where tail connects to the first node.


## 参考答案

/**
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
bool isCycle = false;

while(fp != NULL && sp != NULL){
fp = fp -> next;
if(sp -> next == NULL) return NULL;
sp = sp->next->next;
if(fp == sp) {
isCycle = true;
break;
}
}

if(!isCycle) return NULL;

while(fp != sp) {
fp = fp->next;
sp = sp->next;
}
return fp;

}
};

## 答案注释

posted @ 2019-10-05 19:40  schaffen  阅读(35)  评论(0编辑  收藏