Submission #3272509

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Source Code Expand

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <cmath>
#include <cstdio>
#include <functional>
#include <numeric>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <utility>
#include <sstream>
#include <complex>
#include <fstream>
#include <bitset>
#include <time.h>
#include <tuple>
#include <iomanip>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll, ll> P;
typedef vector<ll> V;
typedef complex<double> Point;

#define PI acos(-1.0)
#define EPS 1e-10
const ll INF = 1e16;
const ll MOD = 1e9 + 7;

#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define rep(i,N) for(int i=0;i<(N);i++)
#define ALL(s) (s).begin(),(s).end()
#define EQ(a,b) (abs((a)-(b))<EPS)
#define EQV(a,b) ( EQ((a).real(), (b).real()) && EQ((a).imag(), (b).imag()) )
#define fi first
#define se second
#define N_SIZE (1LL << 20)
#define NIL -1

ll sq(ll num) { return num*num; }
ll mod_pow(ll x, ll n) {
	if (n == 0)return 1;
	if (n == 1)return x%MOD;
	ll res = sq(mod_pow(x, n / 2));
	res %= MOD;
	if (n % 2 == 1) {
		res *= x;
		res %= MOD;
	}
	return res;
}
ll mod_add(ll a, ll b) { return (a + b) % MOD; }
ll mod_sub(ll a, ll b) { return (a - b + MOD) % MOD; }
ll mod_mul(ll a, ll b) { return a*b % MOD; }

int n;

ll num[1001][1001];
void make_pascal(ll n) {
	num[0][0] = 1;
	rep(i, n+1) {
		rep(j, n+1) {
			if (i != 0)num[i][j] += num[i - 1][j];
			num[i][j] %= MOD;
			if (j != 0)num[i][j] += num[i][j - 1];
			num[i][j] %= MOD;
		}
	}
}
ll nCk_pas(ll n, ll k) {
	return num[n - k][k];
}

vector<int> G[1000];
ll dp[1000][1000];
ll en[1000][1000];

void solve(int now,int to){
	if(dp[now][to] != -1)return;
	dp[now][to] = 1;
	rep(i,G[to].size()){
		int tto = G[to][i];
		if(tto == now)continue;
		solve(to,tto);
		en[now][to] += (en[to][tto]+1);
	}

	ll sum = en[now][to];
	rep(i,G[to].size()){
		int tto = G[to][i];
		if(tto == now)continue;
		dp[now][to] *= dp[to][tto];
		dp[now][to] %= MOD;
		dp[now][to] *= (ll)(nCk_pas(sum,en[to][tto]+1));
		dp[now][to] %= MOD;
		// cout << "!" << sum << " " << en[to][tto] << endl;
		// cout << now << " " << to << " " << dp[now][to] << endl;
		sum -= (en[to][tto]+1);
	}
}

bool f[1000][1000];

int main(){
	cin >> n;
	rep(i,n-1){
		int a,b;
		cin >> a >> b;
		a--; b--;
		G[a].push_back(b);
		G[b].push_back(a);
		f[a][b] = true;
		f[b][a] = true;
	}
	make_pascal(1000);
	rep(i,1000)rep(j,1000)dp[i][j] = -1;
	rep(i,n){
		rep(j,G[i].size()){
			int to = G[i][j];
			if(f[i][to])solve(i,to);
		}
	}
	ll ans = 0;
	rep(i,n){
		rep(j,G[i].size()){
			int to = G[i][j];
			if(i < to){
				ll num = (dp[i][to] * dp[to][i])%MOD;
				num = (num * (ll)(nCk_pas(en[i][to]+en[to][i],en[i][to])))%MOD;
				ans = (ans + num)%MOD;
			}
		}
	}
	cout << ans << endl;
}

Submission Info

Judge Result