bzoj1051 [HAOI2006]受欢迎的牛

#include <cstdio>
#include <stack>
#include <cstring>
#include <iostream>
#include <algorithm>

using namespace std;

const int maxn = 10010, maxm = 50010;

int n, m,head[maxn],to[maxm],nextt[maxm],tot,scc[maxn],dfsclock,low[maxn],pre[maxn],num,shuliang[maxn],ans;
int head2[maxn], to2[maxn], nextt2[maxn], tot2;
stack <int> s;

void add(int x, int y)
{
    tot++;
    to[tot] = y;
    nextt[tot] = head[x];
    head[x] = tot;
}

void add2(int x, int y)
{
    tot2++;
    to2[tot2] = y;
    nextt2[tot2] = head2[x];
    head2[x] = tot2;
}

void tarjan(int u)
{
    s.push(u);
    low[u] = pre[u] = ++dfsclock;
    for (int i = head[u];i;i = nextt[i])
    {
        int v = to[i];
        if (!pre[v])
        {
            tarjan(v);
            low[u] = min(low[u], low[v]);
        }
        else
            if (!scc[v])
                low[u] = min(low[u], pre[v]);
    }
    if (low[u] == pre[u])
    {
        num++;
        while (1)
        {
            int t = s.top();
            s.pop();
            scc[t] = num;
            shuliang[num]++;
            if (t == u)
                break;
        }
    }
}

void lianbian(int u)
{
    for (int i = head[u]; i; i = nextt[i])
    {
        int v = to[i];
        if (scc[u] != scc[v])
            add2(scc[u], scc[v]);
    }
}

int main()
{
    scanf("%d%d", &n, &m);
    for (int i = 1; i <= m; i++)
    {
        int a, b;
        scanf("%d%d", &a, &b);
        add(a, b);
    }
    for (int i = 1; i <= n; i++)
        if (!scc[i])
            tarjan(i);
    for (int i = 1; i <= n; i++)
        lianbian(i);
    for (int i = 1; i <= num; i++)
        if (!head2[i])
        {
            if (ans)
            {
                printf("0\n");
                return 0;
            }
            ans = shuliang[i];
        }
    printf("%d\n", ans);

    return 0;
}