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;
}