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
       
 
       
// Hackerrank: up to 100 digits
       
// Project Euler: 20 digits

       
const unsigned int MaxDigits = 100;
       
 
       
// bonus feature: change maximum sum of three consecutive digits

       
unsigned int maxSum = 9;
       
 
       
// count matching numbers
       
// prevprev: the digit two positions to the left of the current digit
       
// prev    : the digit one position  to the left of the current digit
       
// digits  : number of remainings digits
       
// isFirst : true only for the first digit (which must not be zero)

       
unsigned long long search(unsigned int prevprev, unsigned int prev, unsigned int digits, bool isFirst)
       
{
       
  // done ?
       
  if (digits == 0)
       
    return 1;
       
 
       
  // memoize
       
  unsigned int id = digits * 100 + prevprev * 10 + prev; // simple hash
       
  static unsigned long long cache[(MaxDigits + 1) * 100] = { 0 };
       
  if (cache[id] != 0)
       
    return cache[id];
       
 
       
  // iterate over all digits such that current + prev + prevprev <= 9
       
  unsigned long long result = 0;
       
  for (unsigned int current = 0; current + prev + prevprev <= maxSum; current++)
       
  {
       
    // no leading zero
       
    if (current == 0 && isFirst)
       
      continue;
       
 
       
    // add next digit
       
    result += search(prev, current, digits - 1, false);
       
  }
       
 
       
#ifndef ORIGINAL
       
  // Hackerrank only
       
  result %= 1000000007;
       
#endif
       
 
       
  // memoize
       
  cache[id] = result;
       
  return result;
       
}
       
 
       
int main()
       
{
       
  unsigned int digits = 20;
       
  std::cin >> digits;
       
  if (digits > MaxDigits)
       
    return 1;
       
 
       
#ifdef ORIGINAL
       
  // bonus feature: change maximum sum of three consecutive digits
       
  std::cin >> maxSum;
       
#endif
       
 
       
  std::cout << search(0, 0, digits, true) << std::endl;
       
 
       
  return 0;
       
}

This solution contains 13 empty lines, 16 comments and 6 preprocessor commands.