链表有环检测，环长度，环开始的地方

#include<iostream>
using namespace std;

class node{
public:
    node():value(0),next(NULL){}
    ~node(){}
    int value;
    node* next;
};///be careful this ;

node* createlist(int a[],int n)
{
 node* startnode = new node[n];
 node* ret = startnode;
 for(int i = 0;i<n;i++)
 {
     startnode[i].value = a[i];
     if(i<n-1) 
         startnode[i].next = startnode + i + 1;
 }
 startnode[n-1].next = startnode + 5;
 for(int i = 0;i< 10;i++)
 {
     cout<<" "<<startnode->value;
     startnode = startnode->next;
 }
 cout<<endl;
 return ret;
}

bool iscircle(node* head)
{
    if(NULL == head ||  NULL == head->next)
        return false;
    node* pslow = head;
    node* pfast = head->next;
    while(pfast && pfast->next)
    {
     if(pfast == pslow) return true;
     pslow = pslow->next;
     pfast = pfast->next->next;
    }
    return false;
}


int Length(node* head)
{
    if(NULL == head ||  NULL == head->next)
        return 0;

    node* pslow = head;
    node* pfast = head->next;
    while(pfast && pfast->next)
    {
     if(pfast == pslow) 
     {
      int N = 1;
      pslow = pslow->next;
      while(pfast != pslow)
      {N++;pslow = pslow->next; }
      return N;
     }
     pslow = pslow->next;
     pfast = pfast->next->next;
    }
    return 0;
}

bool nodestartcircle(node* head)///十分注意起始点的选择
{
    if(head == NULL || head->next == NULL) return false;
    node* pfast = head->next;
    node* pslow = head;
    while(pfast && pfast->next)
    {
     if(pfast == pslow) 
     { 
         cout<<pfast->value<<endl;
         pfast = pfast->next;
         pslow = head;
         while(pfast != pslow){pfast = pfast->next;pslow = pslow->next;}
         cout<<pslow->value<<endl;
         return true;
     }
     pslow = pslow->next;
     pfast = pfast->next->next;
    }
    return false;
}


int main()
{
    int a[] = {1,2,3,4,5,6,7,8,9};
    node * t = createlist(a,9);
    //cout<<iscircle(t);
    //Length(t);
    cout<<nodestartcircle(t);
}