判断一个单向链表中是否存在环

#include "iostream"  
#include "map"  
using namespace std;  
map<node*,int>m;  
bool IsLoop(node *head)  
{  
      if(!head)  
           return false;  
      node *p=head;  
      while(p)  
     {  
           if(m[p]==0) // 默认值都是0  
                m[p]=1;  
           else if(m[p]==1)  
                return true;  
           p=p->next;  
     }  
}  

bool IsLoop(node *head)  
{  
    node *pSlow=head;  
    node *pFast=head;  
    while(pSlow!=NULL && pFast!=NULL)  
    {  
        pSlow=pSlow->next;  
        pFast=pFast->next->next;  
        if(pSlow==pFast)  
           return true;  
    }  
    return false;  
}