Leetcode 线性表 Linked List Cycle

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Linked List Cycle

 Total Accepted: 17041 Total
Submissions: 48975

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

Can you solve it without using extra space?

题意：判断一个链表中是否有环

思路：快慢指针，如果有环，最终快慢指针会在非NULL相遇

注：用到fast->next前先要确保fast非NULL,要用fast->next->next前先要确保fast,fast->next非NULL

复杂度：时间O(n)， 空间O(1)

相关题目：Linked List CycleII

/**
* 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) {
ListNode *fast, *slow;
fast = slow = head;
while(fast && fast->next){
fast = fast->next->next;
slow = slow->next;
if(fast == slow) return true;
}
return false;
}