poj2186 强连通缩点

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <algorithm>
#include <string>
#include <vector>
#include <stack>
#include <queue>

const int inf = 0x3f3f3f;
const int MAXN = 1e4+10;
const int MMAXN = 5e4+10;
struct edge{
  int st;
  int to;
  int next;
  int vis;
};

using namespace std;
edge e[MMAXN];
stack<int>s;
int first[MAXN];
int dfn[MAXN];
int low[MAXN];
int mark[MAXN];
int out[MAXN];
int top,sum,cur,ans;
int flag;
int n,m;

void init(){
    memset(first,-1,sizeof(first));
    memset(dfn,0,sizeof(dfn));
    memset(low,0,sizeof(low));
    memset(mark,-1,sizeof(mark));
    memset(out,0,sizeof(out));
    top = sum = cur = flag = ans = 0;
}

void addedge(int u,int v){
    e[top].st = u;
    e[top].to = v;
    e[top].vis = 0;
    e[top].next = first[u];
    first[u] = top++;
}

void dfs(int u,int t){
    dfn[u] = low[u] = t;
    int v;
    s.push(u);
    for(int i=first[u];i!=-1;i=e[i].next){
        v = e[i].to;
        if(!dfn[v]){
            dfs(v,t+1);
            low[u] = min(low[u],low[v]);
        }
        else{
            low[u] = min(low[u],dfn[v]);
        }
    }

    if(low[u]==dfn[u]){
        /*cout<<"********"<<endl;
        cout<<u<<endl;
        cout<<"********"<<endl;*/
        while(s.top()!=u){
            mark[s.top()] = cur;
            s.pop();
        }
        mark[s.top()] = cur;
        s.pop();
        cur++;
    }
}

int main()
{
   // init();
    int a,b,x;
    while(scanf("%d%d",&n,&m)!=EOF){
        init();
        for(int i=0;i<m;i++){
            scanf("%d%d",&a,&b);
            addedge(a,b);
        }
        for(int i=1;i<=n;i++){
            if(!dfn[i])
                dfs(i,1);
        }
        /*for(int i=1;i<=n;i++){
            cout<<mark[i]<<" ";
        }
        cout<<endl;*/
        for(int i=1;i<=n;i++){
            for(int j=first[i];j!=-1;j=e[j].next){
                int v = e[j].to;
                if(mark[i]!=mark[v]){
                    out[mark[i]]++;
                }
            }
        }
        /*for(int i=0;i<cur;i++){
            cout<<out[i]<<" ";
        }
        cout<<endl;*/
        for(int i=0;i<cur;i++){
            if(out[i]==0){
                sum++;
                x = i;
            }
        }
        if(sum!=1){
            cout<<0<<endl;
            continue;
        }
        for(int i=1;i<=n;i++){
            if(mark[i]==x)ans++;
        }
        cout<<ans<<endl;
    }
    //cout << "Hello world!" << endl;
    return 0;
}