1837C - Best Binary String - CodeForces Solution

#include<iostream>
#include<string>
using namespace std;
typedef long long int ll;
int main(){
    int t;
    cin>>t;
    string s;
    while(t--){
        cin>>s;
        ll l = s.length();
        char r[l+1];
        if(s[0] == '?'){
            r[0] = '0';
        }
        else{
            r[0] = s[0];
        }
        for(ll i=1; i<l; i++ ){
            if (s[i] == '?'){
                r[i] = r[i-1];  
            }
            else{
                r[i] = s[i];
            }
        }
        r[l] = '\0';
        cout<<r<<"\n";
    }
}