判断一个链表是不是循环链表

算法思想：在建立循环链表时设置两个指针，头指针和尾指针，head和tail,使tail->next=head,即让尾节点指向头节点，建立的时候采用尾插入法，每次让新节点先指向尾节点的下一个节点，

然后让头节点的下一个节点指向新节点，然后让新节点赋值给头节点和尾节点。

判断是否是循环链表时，也设置两个指针，慢指针和快指针，让快指针比慢指针每次移动快两次。如果快指针追赶上慢指针，则为循环链表，否则不是循环链表，如果快指针或者慢指针指向NULL，则不是循环链表。

include <iostream>
#include <cstdlib>
 
using namespace std;
 
typedef struct list
{
    int data;
    struct list *next;
}loopNode,*lpNode;
 
void createList(lpNode &list,int arr[],int n)     //创建循环链表
 
{
    lpNode node;
    lpNode head=NULL,tail=NULL;
    head=tail=list;
    tail->next=head;
    for(int i=0;i<n;i++)
     {
           node=(struct list*)malloc(sizeof(struct list));
           node->data=arr[i];
           node->next=tail->next;
           head->next=node;
           head=node;
           tail=node;
            
     }
    cout<<"create success"<<endl;
}
 
int judgeIsLoop(lpNode list)      //判断是否是循环链表
   
{
     if(list==NULL)
        return 0;
     lpNode slow,fast;
     slow=list;
     fast=list->next->next;
     while(slow)
   {
     if(fast==NULL||fast->next==NULL)
         return 0;
     else if(fast==slow||fast->next==slow)
         return 1;
      else
          {
            slow=slow->next;
            fast=fast->next->next;
          }
   }
    return 0;
}
void display(lpNode list)     //输出循环链表
{
     lpNode head,tail;
     head=tail=list;
     while(head->next!=tail)
       {
            cout<<head->data<<"--";
            head=head->next;
             
       }
         
       cout<<head->data;
      cout<<endl;
}
 
int main()
{
    lpNode list=NULL;
    int arr[10]={1,2,3,4,5,6,7,8,9,0};
    int flag=0;
    list=(struct list*)malloc(sizeof(struct list));
    list->data=11;
    list->next=NULL;
    createList(list,arr,10);
    flag= judgeIsLoop(list);
    if(flag==1)                                   //如果是循环链表，就输出循环链表
    {
         cout<<"the list is loop list."<<endl;
         cout<<"output the list:"<<endl;
         display(list);
    }
    else
    {
         cout<<"the list is not loop list."<<endl;
    }
    cout<<endl;
    return 0;
     
}