牛客练习赛60 C-操作集锦 长度为k的子序列个数(DP)

题目链接：https://ac.nowcoder.com/acm/contest/4853/C

#include <bits/stdc++.h>
using namespace std;
#define LL long long
const int mod=1e9+7;

char s[100005];
LL  f[1005][1005], g[30]={0};
int main() {

    int n, k;
    scanf("%d%d", &n, &k);
    scanf("%s", s+1);
    for(int i=1; i<=n; i++){
        f[i-1][0]=1;

        for(int j=1; j<=i; j++){
            f[i][j]=(f[i-1][j]+f[i-1][j-1])%mod;
            if(g[s[i]]){
                f[i][j]-=f[g[s[i]]-1][j-1];
            }
            f[i][j]=(f[i][j]+mod)%mod;
        }
        g[s[i]]=i;
    }
    f[n][0]=1;
    cout<<f[n][k]%mod<<endl;

    return 0;
}