tarjan求强连通分量模板

代码：

void tarjan(int u) {
    dfn[u] = ++dfncnt;
    low[u] = dfncnt;
    st.push(u);
    ins[u] = true;
    for (int i = 0; i < SPG[u].size(); i++) {
        int v = SPG[u][i].to;
        if (!dfn[v]) {
            tarjan(v);
            low[u] = min(low[u], low[v]);
        }
        else if (ins[v]) {
            low[u] = min(low[u], dfn[v]);
        }
    }
    if (dfn[u] == low[u]) {
        int v;
        ++scccnt;
        do {
            v = st.top();
            st.pop();
            ins[v] = false;
            scc[v] = scccnt;
        } while (u != v);
    }
}