环形链表

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

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

题意：判断一个链表是否有环

思路：快慢指针，如果会相遇，则说明有环，如果快指针到了链表的末尾没有相遇，则说明没有环

/**
 * 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; 
        ListNode * slow;
        if(head==nullptr || head->next==nullptr) return false;
        //nullptr是c++11为了解决NULL可能被编译为0这个问题，才有的，空指针
        else{
            fast=head;  
            slow=head;
             while(fast->next!=nullptr && fast->next->next!=nullptr)
             {      
                 fast=fast->next->next;
                 slow=slow->next;
                 if(fast==slow) return true;
             }
             return false;
         }
    }
};