Linked List Cycle

一、问题描述

Given a linked list, determine if it has a cycle in it.

Follow up:

Can you solve it without using extra space?

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二、思路

快慢指针，注意考虑边界

三、代码

/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
bool hasCycle(ListNode *head) {
if(head == NULL || head -> next == NULL)
return false;
if(head -> next == head)
return true;
ListNode *fast = head -> next -> next;
ListNode *slow = head -> next;
while(fast != slow && fast != NULL && fast -> next != NULL){
fast = fast -> next -> next;
slow = slow -> next;
if(fast == slow)
return true;
}
return false;
}
};