判断单链表是否有环相关问题（转载加总结）

void Isloop(Llink head)
{
 if(!head||!head->next)
  return;
 Llink p,q;
 bool loop=false;
 p=q=head->next;
 while(q&&q->next)//判断是否有环
 {
  p=p->next;
  q=q->next->next;
  if(p==q)
  {
   loop=true;
   break;
  }
 }
 if(!loop)
  cout<<"This link has not loop\n";
 else
 {
  cout<<"This link has a loop\n";
  Llink r=p;
  q=head->next;
  int nonloop=1,loopcount=1;
  //nonloop计算非环结点数，loopcount计算环上结点数
  do//计算环上的结点数
  {
   p=p->next;
   ++loopcount;
  }while(p!=r);
  --loopcount;
  while(p!=q)//得到环的入口结点，同时计算得到非环的结点数
  {
   p=p->next;
   q=q->next;
   ++nonloop;
  }
  --nonloop;
  cout<<"\nStart of loop: "<<p->data<<endl;  
  cout<<"\nCount of nonloop: "<<nonloop
      <<"\nCount of loop: "<<loopcount
      <<"\nCount of Linknode: "<<nonloop+loopcount<<endl;
 }
}
  
判断是否存在环的程序：
bool IsExitsLoop(slist *head)  
{  
    slist *slow = head, *fast = head;  
    while ( fast && fast->next )   
    {  
        slow = slow->next;  
        fast = fast->next->next;  
        if ( slow == fast ) break;  
    }    
    return !(fast == NULL || fast->next == NULL);  
}  
 
寻找环连接点（入口点）的程序：
slist* FindLoopPort(slist *head)  
{  
    slist *slow = head, *fast = head;    
    while ( fast && fast->next )   
    {  
        slow = slow->next;  
        fast = fast->next->next;  
        if ( slow == fast ) break;  
    }    
    if (fast == NULL || fast->next == NULL)  
        return NULL;  
    slow = head;  
    while (slow != fast)  
    {  
         slow = slow->next;  
         fast = fast->next;  
    }  
    return slow;  
} 
亦可以用类似与hash表的方法，即设立一个数组，将链表结点中的值做数组下标，当赋值冲突时就是环的接入点
 bool isloop(Llink p)
{
 if(!p||!p->next)
  return true;
 int a[MAXSIZE],n=0;
 memset(a,0,sizeof(int)*MAXSIZE);
 p=p->next;
 while(p)
 {
  if(a[p->data]==-1)//存在环时，会发生冲突
  {
   cout<<"\nLoop node: "<<p->data<<endl
    <<"\nLen of node: "<<n<<endl;
   return true;
  }
  a[p->data]=-1;
  ++n;
  p=p->next;
 }
 return false;
}
Llink CreatlinkLoop()
//创建一个有环的链表
{
 Llink head=new Lnode;
 //head->data=0;
 head->next=NULL;
 Lelemtype e;
 Llink q=head;
 int N=0;
 cout<<"input elems:";
 while(cin>>e)
 {
  Llink p=new Lnode;
  ++N;
  p->data=e;
  p->next=q->next;
  q->next=p;
  q=p;
 }
 cin.clear();
 cin.sync();
 srand(time(0));
 q->next=Findnode(head,rand()%N);//随机产生环的接入点
 return head;
}
Llink Findnode(Llink head,int n)//找出链表中的第n个结点
{
 if(n<=0)
  return head;
 Llink p=head->next;
 for(int i=1;p&&i<n;++i)
  p=p->next;
 return p;
}