POJ 2553 The Bottom of a Graph

#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <cstdlib>
#include <algorithm>
#include <vector>
usingnamespace std;
#define INF 0x7ffffff
#define maxn 5005
typedef longlong LL;
#define Min(a,b) (a<b?a:b)
#define MOD 1000000007
int m, n, Time, top, ans;
int Stack[maxn], dfn[maxn], low[maxn], blocks[maxn], Out[maxn];
bool InStack[maxn];
vector<vector<int> > G;
void init()
{
    memset(Out, 0, sizeof(Out));
    memset(dfn, 0, sizeof(dfn));
    memset(low, 0, sizeof(low));
    ans = Time = top = 0;
    G.clear();
    G.resize(n+2);
}
void Tarjan(int u)
{
    dfn[u] = low[u] = ++Time;
    Stack[top++] = u;
    InStack[u] = true;
    int len = G[u].size(), v;

    for(int i=0; i<len; i++)
    {
        v = G[u][i];
        if( !low[v] )
        {
            Tarjan(v);
            low[u] = min(low[u], low[v]);
        }
        elseif( InStack[v] )
            low[u] = min(low[u], dfn[v]);
    }
    if(low[u] == dfn[u])
    {
        do
        {
            v  = Stack[--top];
            InStack[v] = false;
            blocks[v] = ans;
        }
        while(u != v);
        ans ++;
    }
}

void solve()
{
    for(int i=1; i<=n; i++)
    {
        if(!low[i])
            Tarjan(i);
    }

    for(int i=1; i<=n; i++)
    {
        int len = G[i].size(), v;
        for(int j=0; j<len; j++)
        {
            v = G[i][j];
            if(blocks[i] != blocks[v])
                Out[blocks[i]]  ++;
        }
    }

    int flag = 0;

    for(int i=1; i<=n; i++)
    {
        if(Out[blocks[i]] == 0 && !flag)
        {
            flag ++;
            printf("%d", i);
        }
        elseif(Out[blocks[i]] == 0)
            printf(" %d", i);
    }
    printf("\n");
}

int main()
{
    while(scanf("%d",&n), n)
    {
        scanf("%d",&m);
        init();
        while(m --)
        {
            int a, b;
            scanf("%d %d",&a, &b);
            G[a].push_back(b);
        }
        solve();
    }
    return0;
}