关于tarjan

void tarjan(ll x) {
	dfncnt++;
	dfn[x] = dfncnt;
	low[x] = dfn[x];
	S.push(x);
	book[x] = 1;
	for (int i = 0; i < V[x].size(); i++) {
		if (!dfn[V[x][i]]) {
			tarjan(V[x][i]);
			low[x] = min(low[x], low[V[x][i]]);
		} else if (book[V[x][i]]) {
			low[x] = min(low[x], low[V[x][i]]);
		}
	}
	if (low[x] == dfn[x]) {
		sccnt++;
		while (S.top() != x) {
			book[S.top()] = 0;
			blt[S.top()] = sccnt;
			S.pop(); 
		}
		book[S.top()] = 0;
		blt[S.top()] = sccnt;
		S.pop(); 
	}
}

low[x] = min(low[x], low[V[x][i]]);

low[x] = min(low[x], dfn[V[x][i]]);