题解

#include<bits/stdc++.h>
using namespace std;
int main(){
    int n;
    string s;
    cin>>n>>s;
    
    long long ans=0;
    int*lc=new int[n];
    int*rw=new int[n];
    
    lc[0]=(s[0]=='C')?1:0;
    for(int i=1;i<n;i++)                                      {
        lc[i]=lc[i-1]+(s[i]=='C'?1:0);
    }
    
    rw[n-1]=(s[n-1]=='W')?1:0;
    for(int i=n-2;i>=0;i--){
        rw[i]=rw[i+1]+(s[i]=='W'?1:0);
    }
    
    for(int i=0;i<n;i++)                          {
        if(s[i]=='O'){
            ans+=(long long)lc[i]*rw[i];
                                                   }
    }
    
    cout<<ans<<endl;
    
    delete[] lc;
    delete[] rw;
    
    return 0;
}