使用Tarjan进行缩点（有向图）

int From[maxn], Laxt[maxn], To[maxn << 2], Next[maxn << 2], cnt;
int low[maxn], dfn[maxn], times, q[maxn], head, scc_cnt, scc[maxn];
bool inst[maxn];
vector G[maxn];
void add(int u, int v)
{
Next[++cnt] = Laxt[u]; From[cnt] = u;
Laxt[u] = cnt; To[cnt] = v;
}
void tarjan(int u)
{
dfn[u] = low[u] = ++times;
q[++head] = u;
inst[u] = 1;
for (int i = Laxt[u]; i; i = Next[i]) {
if (!dfn[To[i]]) {
tarjan(To[i]);
low[u] = min(low[u], low[To[i]]);
} else if (inst[To[i]]) {
low[u] = min(low[u], dfn[To[i]]);
}
}
if (low[u] == dfn[u]) {
scc_cnt++;
while (true) {
int x = q[head--];
scc[x] = scc_cnt;
inst[x] = 0;
if (x == u) { break; }
}
}
}