判环拓扑

bool dfs(int u)
{
    vis[u] = 1;
    for (int i = head[u]; i; i = edge[i].next) {
        int v = edge[i].to;
        if (vis[v] == 1) return false;
        if (vis[v] == 0 && !dfs(v)) return false;
    }
    vis[u] = -1;
    S.push(u);
    return true;
}