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?
*/

//Unit Test
//
//  |1|-|-->|2|-|-->|3|||
//  /|\                |
//   |                \|/
//  |||6|<--|-|5|<--|-|4| 


#include<iostream>
using namespace std;

struct ListNode {
    int val;
    ListNode *next;
    ListNode(int x) : val(x), next(NULL) {}
}; 

class Solution
{
public:
    Solution();
    ~Solution();

    bool hasCycle(ListNode* head){
        if(head==NULL){         //空链表没有环
            return false;
        }

        ListNode* fast = head;  //快指针
        ListNode* slow = head;  //慢指针

        while(fast->next!=NULL){    //一个节点没有环

            fast=fast->next->next;  //快指针的步长是2
            if(fast == NULL){       //两个节点没有环
                return false;
            }

            slow = slow->next;

            if(fast == slow){
                return true;
            }
        }

        return false;
    }

};

int main(){
    ListNode node1(1);
    ListNode node2(2);
    ListNode node3(3);
    ListNode node4(4);
    ListNode node5(5);
    ListNode node6(6);

    node1.next = &node2;
    node2.next = &node3;
    node3.next = &node4;
    node4.next = &node5;
    node5.next = &node6;
    node6.next = &node1;

    Solution s;
    cout<<boolalpha<<s.hasCycle(&node1)<<endl;  //使用boolalpha输出为bool类型

    return 0;

}