Submission #5901884

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Source Code Expand

#include <bits/stdc++.h>
using namespace std;
#define FOR(i,a,b) for(int i=(a),i##formax=(b);i<i##formax;i++)
#define FORR(i,a,b) for(int i=(a),i##formax=(b);i>=i##formax;i--)
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define pcnt __builtin_popcount
#define sz(x) (int)(x).size()
#define maxs(x,y) x=max(x,y)
#define mins(x,y) x=min(x,y)
#define show(x) cout<<#x<<" = "<<x<<endl;
#define all(a) (a.begin()),(a.end())
#define each(it,c) for(__typeof((c).begin()) it=(c).begin();it!=(c).end();it++)
#define perm(c) sort(all(c));for(bool c##p=1;c##p;c##p=next_permutation(all(c)))
#define bitComb(a,n,k) for(int a##x,a##y,a=(1<<k)-1;a<(1<<n);a##x=a&-a,a##y=a+a##x,a=(((a&~a##y)/a##x)>>1)|a##y)
#define uniq(v) sort(all(v));v.erase(unique(all(v)), v.end())
#define bit(n) (1LL<<(n))
#define randInt(l,r) (uniform_int_distribution<ll>(l,r)(rnd))
#define randDouble(l,r) (uniform_real_distribution<double>(l,r)(rnd))
#define dout(d) printf("%.12f\n",d)

typedef long long ll;
typedef __int128_t lll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
template<class T>using V=vector<T>;
template<class T>using VV=V<V<T>>;
template<class T,class Y>ostream& operator<<(ostream& o,const pair<T,Y>& p){return o<<"("<<p.fi<<", "<<p.se<<")";}
template<typename A,size_t N,typename T>void Fill(A (&array)[N],const T&v){fill((T*)array,(T*)(array+N),v);}
lll gcd(lll a,lll b,lll &x,lll &y){if(!b){x=1;y=0;return a;}lll d=gcd(b,a%b,y,x);y-=a/b*x;return d;}
ll gcd(ll a,ll b){lll x=0,y=0;return gcd(a,b,x,y);}
ll modInv(ll a,ll m){lll x,y;gcd(a,m,x,y);return (x%m+m)%m;}
ll modPow(lll a,lll n,ll m){if(!a)return a;lll p=1;for(;n>0;n>>=1,a=a*a%m)if(n&1)p=p*a%m;return(ll)p;}
bool isPrime(ll n){if(n<2||n%2==0)return n==2;lll t=n-1,d=t/(t&-t);for(lll a:{2,325,9375,28178,450775,9780504,1795265022})if(a%n){for(t=d,a=modPow(a,t,n);t!=n-1&&a!=1&&a!=n-1;a=a*a%n,t=t*2%n);if(a!=n-1&&t%2==0)return 0;}return 1;}
const int IINF = 1e9+6;
const ll LINF = 1e18;
const int MOD = 1e9+7;
const double PI = acos(-1);
const double EPS = 1e-10;
static random_device rd;
static mt19937 rnd(rd());

const int N = 1e3;
map<int, pll> DP[N];
V<int> e[N];
ll f[N+1], r[N+1], ans;
int n;
pll dfs1(int a, int p){
  ll c = 0, s = 1;
  for(int b:e[a]) if(p!=b){
    pll p = dfs1(b, a);
    c += p.fi;
    (s *= r[p.fi]*p.se%MOD) %= MOD;
  }
  (s *= f[c]) %= MOD;
  c++;
  return DP[a][p] = pll(c, s);
}
void dfs2(int a, int p, ll _s){
  ll c, s;
  tie(c, s) = DP[a][p];
  (s *= r[c-1]*r[n-c]%MOD*_s%MOD) %= MOD;
  for(int b:e[a]) if(p!=b){
    ll __c, __s;
    tie(__c, __s) = DP[b][a];
    ans += s*__c%MOD;
    dfs2(b, a, s*f[__c]%MOD*modInv(__s, MOD)%MOD*f[n-__c-1]%MOD);
  }
}
main(){
  cin.tie(0);
  ios::sync_with_stdio(false);
  f[0] = 1; FOR(i, 1, N) f[i] = f[i-1]*i%MOD;
  r[N-1] = modInv(f[N-1], MOD); FORR(i, N-1, 1) r[i-1] = r[i]*i%MOD;
  cin >> n;
  FOR(i, 1, n){
    int a, b;
    cin >> a >> b;
    a--; b--;
    e[a].pb(b);
    e[b].pb(a);
  }
  dfs1(0, -1);
  dfs2(0, -1, 1);
  cout << ans%MOD*f[n-2]%MOD << endl;
}

Submission Info

Judge Result