Submission #994252

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Source Code Expand

#include <iostream>
#include <fstream>
#include <cstdio>
#include <cmath>
#include <vector>
#include <cstring>
#include <string>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <algorithm>
using namespace std;
 
#define REP(i,n) for(int i=0; i<n; ++i)
#define FOR(i,a,b) for(int i=a; i<=b; ++i)
#define FORR(i,a,b) for (int i=a; i>=b; --i)
#define pi M_PI
 
typedef long long ll;
typedef vector<int> VI;
typedef vector<ll> VL;
typedef vector<VI> VVI;
typedef pair<int,int> P;
typedef pair<ll,ll> PL;

#define mod 1000000007

ll add(ll x, ll y){
    return (x+y)%mod;
}

ll sub(ll x, ll y){
    return (x+mod-y) % mod;
}

ll mul(ll x, ll y){
    return (x%mod)*(y%mod)%mod;
}

ll powll(ll x, ll y){
    ll res = 1LL;
    while(y){
        if (y & 1LL)
            res *= x;
        res %= mod;
        x = (x*x) % mod;
        y >>= 1LL;
    }
    return res;
}

ll div(ll x, ll y){
    return (x * powll(y,mod-2)) % mod;
}

ll nPr(ll n, ll r){
    ll res = 1LL;
    FOR(i,1,r){
        res = (res * ((n-i+1) % mod)) % mod;
    }
    return res;
}

ll nCr(ll n, ll r){
    ll res = nPr(n,r);
    FOR(i,1,r){
        res = (res * powll(i,mod-2)) % mod;
    }
    return res;
}

int main() {
    ll n, m;
    cin >> n >> m;
    if (m < n){
        cout << 0 << endl;
        return 0;
    }
    VL p(n);
    REP(i,n){
        p[i] = mul(powll(n-i,n-1),nCr(n-1,i));
    }
    ll ans = 0;
    REP(k,m-n+1){
        ll x = 0;
        REP(i,n){
            if (i % 2 == 0) x = add(x,p[i]);
            else x = sub(x,p[i]);
        }
        x = mul(x, powll(n-1,m-n-k));
        ans = add(ans, x);

        REP(i,n){
            p[i] = mul(p[i], n-i);
        }
    }

    cout << -1 << endl;

    return 0;
}

Submission Info

Judge Result