Reverse a string using no variables while using recursion in c++

Question

How can you reverse a string using recursion following the constraints mentioned in the function

string reverse_recurse(string forward) { // recursive
// Constraints: No loops allowed; no static local or global variables.
// Your new code goes here...
return ""; 
}

Answer 1

Here is a quick implementation of the second approach described in Robᵩ's answer. Basically, return the last element of the string concatenated with the reverse of the other elements.

#include <iostream>
#include <string>

std::string reverse_string(std::string a_string) {
    return
      a_string.length() > 0 ?
        a_string.back()
          + reverse_string(a_string.substr(0, a_string.length() - 1)) :
        std::string();
}

int main() {
    std::string a_string = "live on time , emit no evil";

    std::cout << a_string << std::endl;
    std::cout << reverse_string(a_string) << std::endl;

    return 0;
}