【YCOJ 3805】竞选

#include<cstdio>
#include<iostream>
#include<cmath>
#include<cstdlib>
#include<cstring>
#include<algorithm>
typedef long long ll;
using namespace std;
const int N=1000003;
const int mod=1000000007;
int ans,n,k,a[N],tot;
char s[N];
int main(){
    
    scanf("%d %d",&n,&k);
    a[0]=1; 
    for(int i=1;i<=n;i++) 
        a[i]=(a[i-1]<<1)%mod;//cout<<a[i]<<endl;
    scanf("%s",s+1);
    for(int i=n;i>=1;i--){
        if(s[i]==‘B‘){
            //cout<<i<<tot<<ans<<endl;
            ++tot;
            if(tot>k){
                ans=(ans+a[i])%mod;
                tot--;
            }
            else continue;
        }
        tot=max(0,tot-1);
    }
    printf("%d\n",ans%mod);
    return 0;
}