繁华模拟赛 Vincent的城堡

Posted 2020-08-10 ACforever

#include<iostream>
#include<cstdio>
#include<string>
#include<cstring>
#include<algorithm>
#define ll long long
using namespace std;
const ll mod = 1000000007;
ll n,k,c[20][20];
ll ansa,ansb,ans;
void get_c(){
    for(int i = 1;i <= 10;i++){
        c[i][0] = c[i][i] = 1;
        c[i][1] = c[i][i-1] = i;
    }
    for(int i = 2;i <= 10;i++){
        for(int j = 2;j < i;j++){
            c[i][j] = c[i-1][j-1] + c[i-1][j];
        }
    }
}
ll q_mul(ll a,ll b){
    ll ans = 0;
    while(b){
        if(b&1){
            ans = (ans + a) % mod;
        }
        a = (a + a) % mod;
        b >>= 1;
    }
    return ans;
}
ll q_pow(ll a,ll b){
    ll ans = 1;
    while(b){
        if(b&1){
            ans = q_mul(ans,a);
        }
        a = q_mul(a,a);
        b >>= 1;
    }
    return ans;
}
int main(){
    freopen("castle.in","r",stdin);
    freopen("castle.out","w",stdout);
    cin>>n>>k;
    get_c();
    ansb = q_pow(n-k,n-k);
    ansa = q_pow(k,k-1);//诡异结论
    ans = q_mul(ansa,ansb);
    cout<<ans;
    return 0;
}