操作集锦

https://ac.nowcoder.com/acm/contest/4853/C

 

#include <bits/stdc++.h>
using namespace std;
#define int long long

const int maxn = 1e3 + 5;
const int mod = 1e9 + 7;

int dp[maxn][maxn];

int n, k, pre[maxn];
char s[maxn ];

signed main(){
    cin >> n >> k;
    scanf("%s", s + 1);
    dp[0][0] = 1;
    for (int i = 1; i <= n; ++i) {
        dp[i][0] = 1;
        for (int j = 1; j <= i; ++j) {
            dp[i][j] = dp[i - 1][j] + dp[i - 1][j - 1];
            if (pre[s[i] - 'a'])dp[i][j] -= dp[pre[s[i] - 'a'] - 1][j - 1];
            dp[i][j] %= mod;
        }
        pre[s[i] - 'a'] = i;
    }
    int  res = dp[n][k];
    if (res < 0)res += mod;
    printf("%lld
", res);
    return 0;
}