Wannafly挑战赛5 D. 子序列

D. 子序列

链接

#include<cstdio>
#include<algorithm>
#include<cstring>
#include<iostream>
#include<cmath>
#include<cctype>
#include<set>
#include<queue>
#include<vector>
#include<map>
using namespace std;
typedef long long LL;

inline int read() {
    int x=,f=;char ch=getchar();for(;!isdigit(ch);ch=getchar())if(ch=='-')f=-;
    for(;isdigit(ch);ch=getchar())x=x*+ch-'';return x*f;
}

const int N = ;
char s[N];
int fac[N], ifac[N];
const int mod = 1e9 + ;

int ksm(int a,int b) {
    int res = ;
    while (b) {
        if (b & ) res = 1ll * res * a % mod;
        a = 1ll * a * a % mod;
        b >>= ;
    }
    return res;
}
int C(int n,int m) {
    return 1ll * fac[n] * ifac[m] % mod * ifac[n - m] % mod;
}

int main() {
    fac[] = ;
    for (int i = ; i <= ; ++i) fac[i] = 1ll * fac[i - ] * i % mod;
    for (int i = ; i <= ; ++i) ifac[i] = ksm(fac[i], mod - );

    scanf("%s", s + );
    int m = read(), n = strlen(s + );
    LL Ans = ;
    for (int i = n; i <= m; ++i) {
        Ans += 1ll * C(i - , n - ) * ksm(, i - n) % mod * ksm(, m - i) % mod;
        Ans %= mod;
    }
    cout << (Ans + mod) % mod;
    return ;
}