141. 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?

翻译：

给定一个链表，确定链表是否有一个循环

分析：快慢指针解题

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