Leetcode[141]-Linked List Cycle

Link:https://leetcode.com/problems/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?

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分析：设置两个临时指针，一个一次走一步，一个一次走两步，如果再次相遇，表示有环。

Code(c++):

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