NOIP2015 信息传递

题目

vijos1979

代码

include

include

include

include

include

include

include

include

define N 200005

define inf 2000000000

using namespace std;

int n,sum[N],ans=inf;
int t,dfn[N],low[N],be[N];
vector to[N];
stack st;

int cnt;bool f[N];
void Tarjan(int x)
{
dfn[x]=low[x]=++t;
st.push(x);f[x]=1;
for(vector::iterator it=to[x].begin();it!=to[x].end();it++)
{
if(!dfn[it])
Tarjan(it),low[x]=min(low[x],low[it]);
else if(f[it]) low[x]=min(low[x],dfn[*it]);
}
if(low[x]==dfn[x])
{
cnt++;
while(f[x])
{
int k=st.top();st.pop();
f[k]=0;be[k]=cnt;
}
}
}

int main()
{
scanf("%d",&n);
for(int i=1;i<=n;i++)
{
int x;scanf("%d",&x);to[i].push_back(x);
}
for(int i=1;i<=n;i++)
if(!dfn[i]) Tarjan(i);
for(int i=1;i<=n;i++) sum[be[i]]++;
for(int i=1;i<=cnt;i++)
if(sum[i]>1) ans=min(ans,sum[i]);
printf("%d",ans);
return 0;
}