P1365 WJMZBMR打osu! / Easy

#include<bits/stdc++.h>
using namespace std;
const int N=3e5+100;
long double len,f[N];
string s;
int l;
int main(){
	cin>>l>>s;
	for(int i=1;i<=l;i++)
		switch(s[i-1]){
			case 'x':
				f[i]=f[i-1];
				len=0;
				break;
			case 'o':
				f[i]=f[i-1]+2*len+1;
				len++;
				break;
			case '?':
				f[i]=f[i-1]+len+0.5;
				len=(len+1)/2;
				break;
		}
	printf("%.4Lf",f[l]);
}