Leetcode 41. 环形链表（简单）linked-list-cycle——链表、判断是否循环链表、快慢指针

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

Follow up:
Can you solve it without using extra space?

若在while开始时判断fast==slow，会出现误判，即第一次循环时fast必定等于slow

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