My code

… was written in C++11 and can be compiled with G++, Clang++, Visual C++. You can download it, too. Or just jump to my GitHub repository.

The code contains #ifdefs to switch between the original problem and the Hackerrank version.
Enable #ifdef ORIGINAL to produce the result for the original problem (default setting for most problems).

       
#include <iostream>
       
 
       
//#define ORIGINAL

       
 
       
// find all numbers where x^digits has digits digits (I'm loving that comment ...)

       
unsigned int check(unsigned int digits)
       
{
       
  // numbe rof matches
       
  unsigned int count = 0;
       
 
       
  // unsigned long long isn't sufficient for the highest digits
       
  // a double has some rounding issues but they don't affect to result
       
#ifdef ORIGINAL
       
typedef double Number;
       
#else
       
typedef unsigned long long Number;
       
#endif
       
 
       
  // range of valid numbers
       
  // from = 10^(digits-1)
       
  // to   = 10^digits - 1
       
  Number to = 1;
       
  for (unsigned int i = 1; i <= digits; i++)
       
    to *= 10;
       
  Number from = to / 10;
       
  to--;
       
 
       
  // try all single-digit base numbers
       
  for (unsigned int base = 1; base <= 9; base++)
       
  {
       
    // compute power = base ^ digits
       
    Number power = base;
       
    for (unsigned int i = 1; i < digits && power <= to; i++)
       
      power *= base;
       
    // could use C++'s pow(), too
       
 
       
    // right number of digits ?
       
    if (power >= from && power <= to)
       
    {
       
      count++;
       
#ifndef ORIGINAL
       
      std::cout << std::fixed << power << std::endl;
       
#endif
       
    }
       
  }
       
 
       
  return count;
       
}
       
 
       
int main()
       
{
       
#ifdef ORIGINAL
       
  // check all digits
       
  unsigned int count = 0;
       
  for (unsigned int digits = 1; digits <= 21; digits++) // I observed no results with more than 21 digits
       
    count += check(digits);
       
  std::cout << count << std::endl;
       
 
       
#else
       
 
       
  // check only certain digits
       
  unsigned int digits = 9;
       
  std::cin >> digits;
       
  check(digits);
       
#endif
       
 
       
  return 0;
       
}

This solution contains 11 empty lines, 14 comments and 9 preprocessor commands.