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>
       
#include <vector>
       
 
       
// print result modulo some value

       
#define ORIGINAL
       
#ifdef ORIGINAL
       
const unsigned long long Modulo = 1000000000000000000ULL; // high enough to keep the result unchanged
       
#else
       
const unsigned long long Modulo = 1000000007ULL;
       
#endif
       
 
       
int main()
       
{
       
  // Googol = 100, but Hackerrank wants more ...
       
  const unsigned int numDigits = 100000;
       
  std::vector<unsigned long long> solutions(numDigits + 1, 0);
       
 
       
  // count how many numbers are increasing and/or decreasing
       
  typedef unsigned long long DigitCounter[10]; // some older GCC complain about using this in a vector
       
  std::vector<DigitCounter> increase(numDigits);
       
  std::vector<DigitCounter> decrease(numDigits);
       
 
       
  // all one-digit numbers are non-bouncy
       
  unsigned long long sum = 9;
       
  for (auto& x : increase[0])
       
    x = 1;
       
  for (auto& x : decrease[0])
       
    x = 1;
       
 
       
  // process digits, beginning from the right side
       
  for (unsigned int i = 1; i < numDigits; i++)
       
  {
       
    // digits 0..9
       
    for (unsigned int current = 0; current <= 9; current++)
       
    {
       
      // add count of all numbers where the next digit is equal or lower
       
      decrease[i][current] = 0;
       
      for (unsigned int smaller = 0; smaller <= current; smaller++)
       
        decrease[i][current] = (decrease[i][current] + decrease[i - 1][smaller]) % Modulo;
       
 
       
      // add count of all numbers where the next digit is equal or higher
       
      increase[i][current] = 0;
       
      for (unsigned int bigger = current; bigger <= 9; bigger++)
       
        increase[i][current] = (increase[i][current] + increase[i - 1][bigger]) % Modulo;
       
    }
       
 
       
    // compute total sum of increasing and decreasing numbers
       
    for (auto x : increase[i])
       
      sum += x;
       
    for (auto x : decrease[i])
       
      sum += x;
       
 
       
    // but numbers must not start with zero !
       
    sum -= increase[i][0];
       
 
       
    // numbers with identical digits were counted twice
       
    // because they are both increasing and decreasing (e.g. 55555555)
       
    sum -= 10;
       
 
       
    // Hackerrank only
       
    sum %= Modulo;
       
    solutions[i] = sum;
       
  }
       
 
       
  // lookup results
       
  unsigned int tests;
       
  std::cin >> tests;
       
  while (tests--)
       
  {
       
    unsigned int digits;
       
    std::cin >> digits;
       
    std::cout << solutions[digits - 1] << std::endl; // 0-based array but 1-based input
       
  }
       
 
       
  return 0;
       
}

This solution contains 12 empty lines, 14 comments and 6 preprocessor commands.