Hdu 4661 树上拓扑序计数

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int MAXN = 1000005;
const int MAXM = 1000005;
const int mod = 1e9 + 7;
int to[MAXM << 1], nxt[MAXM << 1], Head[MAXN], ed = 1;
inline void addedge(int u, int v) {
        to[++ed] = v;
        nxt[ed] = Head[u];
        Head[u] = ed;
}
int T, n;
ll dp[MAXN], sz[MAXN];
ll fac[MAXN], inv[MAXN];
ll anser = 0;
ll Qpow(ll a, ll b) {
        ll ans = 1, base = a;
        while (b != 0) {
                if (b & 1 != 0) {
                        ans *= base;
                        ans %= mod;
                }
                base *= base;
                base %= mod;
                b >>= 1LL;
        }
        return ans;
}
void init() {
        fac[0] = 1;
        for (ll i = 1; i <= 1000000; i++) {
                fac[i] = fac[i - 1] * i % mod;
        }
        for (int i = 1; i <= 1000000; i++) {
                inv[i] = Qpow(fac[i], mod - 2);
        }
}
void getsz(int x, int fa) {
        sz[x] = 1;
        for (int v, i = Head[x]; i; i = nxt[i]) {
                v = to[i];
                if (v == fa) {
                        continue;
                }
                getsz(v, x);
                sz[x] += sz[v];
        }
        return ;
}
void dp1(int x, int fa) {
        dp[x] = 1;
        for (int v, i = Head[x]; i; i = nxt[i]) {
                v = to[i];
                if (v == fa) {
                        continue;
                }
                dp1(v, x);
                dp[x] = ((dp[x] * dp[v]) % mod * inv[sz[v]]) % mod;
        }
        dp[x] = dp[x] * fac[sz[x] - 1] % mod;
        return ;
}
void dp2(int x, int fa) {
        if (x != 1) {
                //dp[x] = dp[fa] * sz[x] % mod * inv[n - sz[x]] % mod;
                dp[x] = dp[fa] * sz[x] % mod * Qpow(n - sz[x], mod - 2) % mod;
                anser = (anser + dp[x] * dp[x] % mod) % mod;
        }
        for (int v, i = Head[x]; i; i = nxt[i]) {
                v = to[i];
                if (v == fa) {
                        continue;
                }
                dp2(v, x);
        }
        return ;
}
int main() {
        init();
        int u, v;
        scanf("%d", &T);
        while (T--) {
                anser = 0;
                scanf("%d", &n);
                ed = 1;
                for (int i = 1; i <= n; i++) {
                        Head[i] = 0;
                }
                for (int i = 1; i < n; i++) {
                        scanf("%d %d", &u, &v);
                        addedge(u, v), addedge(v, u);
                }
                getsz(1, 0);
                dp1(1, 0);
                anser = dp[1] * dp[1] % mod;
                dp2(1, 0);
                anser += mod;
                anser %= mod;
                cout << anser << endl;
        }
        return 0;
}