void tarjan(int u)
{
    dfn[u] = low[u] = ++idx;
    st[top++] = u;
    for (Edge cur : G[u])
        if (!dfn[cur.to])
            tarjan(cur.to),
            low[u] = min(low[u], low[cur.to]);
        else if (!scc[cur.to])
            low[u] = min(low[u], dfn[cur.to]);
    if (dfn[u] == low[u] && ++cnt)
        do scc[st[--top]] = cnt;
        while (st[top] != u);
}