The spy life || won't pass test 4

Question

Don't know why it only fails test4.
Can someone help me figure out what I am missing?

#include <iostream>
#include <string>
using namespace std;

int main(){

string code;
string sol;

getline(cin,code);

int n=0;
int s = code.length ();

for(int i=0;i<code.size();i++){
if((code[i]>=97&&code[i]<=122)||code[i]==32||(code[i]>=65&&code[i]<=90)){
sol[n]=code[i];
n++;
}else{s--;}
}

for(int i=s-1;i>=0;i--){
cout<<sol[i];}

return 0;
}

Answer 1

In sol[n] = code[i] use sol += code [i] instead.