牛客多校Round 9

Solved:1

rank:112

E. Music Game

题解说有个非简化的原题 bzoj4318

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll mod = 1e9 + 7;
ll n, m;
ll p[1005];
ll comb[1005][1005];

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


int main()
{
    scanf("%lld%lld", &n, &m);
    ll inv = pow_mod(100, mod - 2);
    for(int i = 1; i <= n; i++) scanf("%lld", &p[i]);
    for(int i = 1; i <= n; i++)
    {
        comb[i][i] = p[i] * inv % mod;
        for(int j = i + 1; j <= n; j++) comb[i][j] = comb[i][j - 1] * p[j] % mod * inv % mod;
    }

    ll ans = 0;
    for(int i = 1; i <= n; i++)
    {
        for(int j = i; j <= n; j++)
        {
            ll tmp = pow_mod(j - i + 1, m);
            (tmp *= comb[i][j]) %= mod;
            if(i != 1) tmp = tmp * (100 - p[i - 1]) % mod * inv % mod, tmp %= mod;
            if(j != n) tmp = tmp * (100 - p[j + 1]) % mod * inv % mod, tmp %= mod;
            (ans += tmp) %= mod;
        }
    }
    printf("%lld
", ans);
    return 0;    
}