POJ 2553 The Bottom of a Graph

#include<iostream>
#include<cstdio>
#include<vector>
#include<stack>
#include<cstring>
using namespace std;
int n,m;
int book[50008];
int low[50008],num[50008],cnt=1,index;
int color[50008];
bool flag[50008];
vector<int>u[50008];
stack<int>st;
int sig=0;
void Tarjan(int t)
{
    num[t]=low[t]=++index;
    st.push(t);
    book[t]=true;
    int siz=u[t].size();
    for(int i=0;i<siz;i++){
        if(!num[u[t][i]]){
            Tarjan(u[t][i]);
            low[t]=min(low[t],low[u[t][i]]);
        }
        else if(book[u[t][i]]){low[t]=min(low[t],low[u[t][i]]);}
    }

    if(num[t]==low[t]){
        sig++;
        while(1){

            cnt=st.top();
            st.pop();
            color[cnt]=sig;
            book[cnt]=0;
            if(cnt==t){break;}
        }
    }
}


bool init()
{
    scanf("%d",&n);
    for(int i=1;i<=n;i++){
        u[i].clear();
    }
    while(!st.empty()){
        st.pop();
    }
    memset(book,0,sizeof(book));
    memset(low,0,sizeof(low));
    memset(flag,0,sizeof(flag));
    memset(color,0,sizeof(color));
    memset(num,0,sizeof(num));
    index=0;
    if(n==0){return false;}
    scanf("%d",&m);
    int x,y;
    for(int i=1;i<=m;i++){
        scanf("%d%d",&x,&y);
        u[x].push_back(y);
    }
    return true;
}


void solve()
{
    int siz;
    int tle=0;
    for(int i=1;i<=n;i++){
        siz=u[i].size();
        for(int j=0;j<siz;j++){
            if(color[u[i][j]]!=color[i]){flag[color[i]]=true;}
        }
    }

    for(int i=1;i<=n;i++){
        if(!flag[color[i]]){
            tle++?printf(" %d",i):printf("%d",i);
        }
    }
    printf("\n");
}

int main()
{
    while(init()){
        for(int i=1;i<=n;i++){
            if(!num[i]){Tarjan(i);cnt++;}
        }
        solve();
    }
}