#include <iostream>
#include <string>

void sub(std::string str, int start, int size){
    if(size == 0){
        return;
    }
    std::cout<<str[start]<<' ';
    sub(str, start+1, size-1);
}

int main() {
    std::string str = "abc";
    int size = str.size();
    sub(str, 0, size);
}