怎么办呢……

#include<iostream>
using namespace std;
string s;
string fbi(int l,int r,char &g){
    if(l==r){
        if(s[l]=='l')return "I";
            else return "B";
    }
    char zg,yg;
    string zh=fbi(l,(l+r)/2,zg);
    string yh=fbi((l+r)/2+1,r,yg);
    if(zg==yg){g=zg;}
        else g='F';
    return zh+yh+g;
}

int main(){
    int n;
    char g;
    cin>>n;
    cin>>s;
    cout<<fbi(0,s.size()-1,g);

    return 0;
}