poj 3177 Redundant Paths tarjan

#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;
const int maxn=5e3+9;
struct
{
int to,next;
}e[maxn<<2];
int head[maxn],lon;
void edgeini()
{
memset(head,-1,sizeof(head));
lon=-1;
}
void edgemake(int from,int to)
{
e[++lon].to=to;
e[lon].next=head[from];
head[from]=lon;
}
int dfn[maxn],low[maxn],instack[maxn],count,stack[maxn],top;
int ans;
void tarjan(int t,int flag,int from)
{
int kk=0;
dfn[t]=low[t]=++count;
instack[t]=1;
stack[++top]=t;
for(int k=head[t];k!=-1;k=e[k].next)
{
if(k==(flag^1)) continue;
int u=e[k].to;
if(dfn[u]==-1)
{
++kk;
tarjan(u,k,t);
low[t]=min(low[t],low[u]);
}
else if(instack[u])
{
low[t]=min(low[t],dfn[u]);
}
}
if(low[t]==dfn[t])
{
int cnt=top;
while(stack[top]!=t)
{
instack[stack[top--]]=0;
}
instack[stack[top--]]=0;
if(cnt-top+low[t]-1==count)
{
ans++;
}
else if(t==from&&kk==1)
ans++;
}
}

int main()
{
// freopen("in.txt","r",stdin);
int n,m;
while(scanf("%d %d",&n,&m)!=EOF)
{
edgeini();
for(int i=1,from,to;i<=m;i++)
{
scanf("%d %d",&from,&to);
edgemake(from,to);
edgemake(to,from);
}
memset(dfn,-1,sizeof(dfn));
memset(instack,0,sizeof(instack));
count=top=ans=0;
tarjan(1,-1,1);
if(ans==1)
printf("0\n");
else
printf("%d\n",(ans+1)/2);
}
return 0;
}