【模板】缩点

int n;
int h[MAX_N], nxt[MAX_N * MAX_N], to[MAX_N * MAX_N], tot;
int c[MAX_N], cnt;
int dfn[MAX_N], low[MAX_N];
int st[MAX_N], top;
int vis[MAX_N];
int dep;

void Tarjan(int x)
{
    dfn[x] = low[x] = ++dep;
    st[++top] = x;
    vis[x] = 1;
    for(register int i = h[x]; i; i = nxt[i])
    {
        if(!dfn[to[i]]) Tarjan(to[i]);
        else if(vis[to[i]] ^ 1) continue;
        low[x] = min(low[x], low[to[i]]);
    }
    if(dfn[x] == low[x])
    {
        c[x] = ++cnt;
        vis[x] = 0;
        while(st[top] != x)
        {
            c[st[top]] = cnt;
            vis[st[top--]] = 0;
        }
        --top;
    }
    return;
}

int main()
{
    // input.....
    for(register int i = 1; i <= n; ++i)
    {
        if(c[i]) continue;
        Tarjan(i);
    }
    // program.....
    return 0;
}