【题解】
期望题。设f[i]为到第i位的期望得分，g[i]为到第i位的期望combo.
若s[i]为o，f[i]=f[i-1]+2*g[i-1]+1(也就是f[i]-f[i-1]=(g[i-1]+1)^2-g[i-1]^2), g[i]=g[i-1]+1;
若s[i]为x，f[i]=f[i-1], g[i]=0;
若s[i]为?，f[i]=0.5*f[i-1]+0.5*(f[i-1]+2*g[i-1]+1)=f[i-1]+g[i-1]+0.5, g[i]=0.5*(g[i-1]+1)
1 #include<cstdio> 2 #include<cstring> 3 #include<algorithm> 4 #define LL long long 5 #define rg register 6 #define N 500010 7 using namespace std; 8 int n; 9 double f[N],g[N]; 10 char s[N]; 11 int main(){ 12 scanf("%d ",&n); scanf("%s",s+1); 13 for(rg int i=1;i<=n;i++){ 14 if(s[i]=='o') f[i]=f[i-1]+2*g[i-1]+1,g[i]=g[i-1]+1; 15 else if(s[i]=='x') f[i]=f[i-1],g[i]=0; 16 else f[i]=f[i-1]+g[i-1]+0.5,g[i]=0.5*(g[i-1]+1); 17 } 18 printf("%.4lf ",f[n]); 19 return 0; 20 }