牛客练习赛60 C操作集锦(dp 去重)

// Problem: 操作集锦
// Contest: NowCoder
// URL: https://ac.nowcoder.com/acm/contest/4853/C
// Memory Limit: 524288 MB
// Time Limit: 2000 ms
// 
// Powered by CP Editor (https://cpeditor.org)
#include<bits/stdc++.h>
using namespace std;
typedef long long ll;typedef unsigned long long ull;
typedef pair<ll,ll>PLL;typedef pair<int,int>PII;typedef pair<double,double>PDD;
#define I_int ll
inline ll read(){ll x=0,f=1;char ch=getchar();while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}return x*f;}
#define read read()
#define rep(i, a, b) for(int i=(a);i<=(b);++i)
#define dep(i, a, b) for(int i=(a);i>=(b);--i)
ll ksm(ll a,ll b,ll p){ll res=1;while(b){if(b&1)res=res*a%p;a=a*a%p;b>>=1;}return res;}
const int maxn=1100,maxm=1e6+7;
const ll mod=1e9+7;
ll n,k,pre[maxn],pos[maxn],dp[maxn][maxn];
char s[maxn];
int main(){
  n=read,k=read;
  cin>>s+1;
  for(int i=1;i<=n;i++){
    pre[i]=pos[s[i]];
    pos[s[i]]=i;
  }
  dp[0][0]=1;
  for(int i=1;i<=n;i++){
    for(int j=0;j<=i;j++){
      dp[i][j]=(dp[i-1][j]+dp[i][j])%mod;
      if(j>=1) dp[i][j]=(dp[i-1][j-1]+dp[i][j])%mod;
      if(pre[i]>=1) dp[i][j]=(dp[i][j]-dp[pre[i]-1][j-1]+mod)%mod;
    }
  }
  cout<<(dp[n][k]+mod)%mod;
  return 0;
}