POJ 2186 Popular Cows 强连通

#include <cstdlib>
#include <cctype>
#include <cstring>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <vector>
#include <string>
#include <iostream>
#include <sstream>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <fstream>
#include <numeric>
#include <iomanip>
#include <bitset>
#include <list>
#include <stdexcept>
#include <functional>
#include <utility>
#include <ctime>

using namespace std;

#define PB push_back
#define MP make_pair
#define REP(i,n) for(int i=0;i<(n);++i)
#define FOR(i,l,h) for(int i=(l);i<=(h);++i)
#define DWN(i,h,l) for(int i=(h);i>=(l);--i)
#define CLR(vis) memset(vis,0,sizeof(vis))
#define MST(vis,pos) memset(vis,pos,sizeof(vis))
#define MAX3(a,b,c) max(a,max(b,c))
#define MAX4(a,b,c,d) max(max(a,b),max(c,d))
#define MIN3(a,b,c) min(a,min(b,c))
#define MIN4(a,b,c,d) min(min(a,b),min(c,d))
#define PI acos(-1.0)
#define INF 0x7FFFFFFF
#define LINF 1000000000000000000LL
#define eps 1e-8

typedef long long ll;

const int maxn=55555;

struct node{
    int u,to,next;
}e[maxn];

int head[maxn],edge;

int id[maxn],q[maxn],dfn[maxn],low[maxn],num[maxn];

int n,m,tsp,qe,cnt;

void init()
{
    MST(head,-1);
    edge=0;
}

void addedge(int u,int v)
{
    e[edge].u=u;
    e[edge].to=v;
    e[edge].next=head[u];
    head[u]=edge++;
}

void tarjan(int u)
{
    int i,v;
    dfn[u]=low[q[qe++]=u]=++tsp;
    for(i=head[u];i>=0;i=e[i].next)
        if(!dfn[v=e[i].to])
        {
            tarjan(v);
            low[u]=min(low[u],low[v]);
        }
        else
        {
            if(id[v]<0)
            {
                low[u]=min(low[u],dfn[v]);
            }
        }
    if(low[u]==dfn[u])
    {
        num[id[u]=++cnt]=1;
        while((v=q[--qe])!=u)
            ++num[id[v]=cnt];
    }
}

void solve()
{
    int i;
    tsp=qe=cnt=0;
    for(i=0;i<=n;++i)
        id[i]=-1,dfn[i]=0;
    for(i=1;i<=n;++i)
        if(!dfn[i])
            tarjan(i);
}

int main()
{
    while(scanf("%d",&n)!=EOF)
    {
        if(n==0)
           break;
        init();
        scanf("%d",&m);
        int u,v;
        REP(i,m)
        {
            scanf("%d%d",&u,&v);
            addedge(u,v);
        }
        solve();
        int out[maxn];
        CLR(out);
        REP(i,edge)
        {
            u=id[e[i].u],v=id[e[i].to];
            if(u!=v)
                out[u]++;
        }
        int ans=0;
        int res;
        FOR(i,1,cnt)
        {
            if(!out[i])
                ans++,res=i;
        }
        if(ans>1)
            cout<<0<<endl;
        else
            cout<<num[res]<<endl;
    }
    return 0;
}