Print all Codes - String

 #include <string.h>
using namespace std;
 
void helper(string input , string output){
    if(input.empty()){
      //output[0]==" ";
        cout<<output<<endl;
        return;
    }
     char size1 = 'a'+(input[0]-'0')-1;
        helper(input.substr(1),output+size1);
    
    char size2;
    int firstname = 10*(input[0]-'0')+(input[1]-'0');
    
        if(firstname>=10 && firstname<=26){
            size2 = 'a'+firstname -1;
                helper(input.substr(2) , output+size2);
           
        }
 }

void printAllPossibleCodes(string input) {
    string output="";
    helper( input,  output);
    /*
    Given the input as a string, print all its possible combinations. You do not need to return anything.
    */
}