【LeetCode】Linked List Cycle

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

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

思路：设两个快慢指针，慢指针每次走一步，快指针每次做一步，如果是个圈快慢指针总会相遇。否则就是不是一个环状的结构。

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