My code

… was written in C++11 and can be compiled with G++, Clang++, Visual C++. You can download it, as well as the input data, 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 <string>
       
#include <iostream>
       
 
       
typedef unsigned int Board[9][9];
       
const unsigned int Empty = 0;
       
 
       
// find the first solution and store it in board

       
bool solve(Board& board)
       
{
       
  for (unsigned int y = 0; y < 9; y++)
       
    for (unsigned int x = 0; x < 9; x++)
       
    {
       
      // already solved cell ?
       
      if (board[x][y] != Empty)
       
        continue;
       
 
       
      // track which numbers could be placed in the current cell
       
      bool available[9+1] = { false,  true, true, true, true, true, true, true, true, true };
       
      // note: available[0] is a dummy entry, the program only uses available[1..9]
       
 
       
      // same row and column
       
      for (unsigned int i = 0; i < 9; i++)
       
      {
       
        if (board[i][y] != Empty)
       
          available[board[i][y]] = false;
       
        if (board[x][i] != Empty)
       
          available[board[x][i]] = false;
       
      }
       
      // same region (3x3 area)
       
      unsigned int rx = (x / 3) * 3;
       
      unsigned int ry = (y / 3) * 3;
       
      for (unsigned int i = 0; i < 3; i++)
       
        for (unsigned int j = 0; j < 3; j++)
       
          if (board[i + rx][j + ry] != Empty)
       
            available[board[i + rx][j + ry]] = false;
       
 
       
      // try all still available numbers
       
      for (unsigned int i = 1; i <= 9; i++)
       
        if (available[i])
       
        {
       
          board[x][y] = i;
       
          if (solve(board))
       
            return true;
       
        }
       
      // all failed, restore old board
       
      board[x][y] = Empty;
       
      return false;
       
    }
       
 
       
  // solved it
       
  return true;
       
}
       
 
       
int main()
       
{
       
//#define ORIGINAL

       
#ifdef ORIGINAL
       
  unsigned int tests = 50;
       
  unsigned int sum   =  0;
       
#else
       
  unsigned int tests =  1;
       
#endif
       
 
       
  while (tests--)
       
  {
       
#ifdef ORIGINAL
       
    // skip labels "GRID xy"
       
    std::string dummy;
       
    std::cin >> dummy >> dummy;
       
#endif
       
 
       
    // read board
       
    Board board;
       
    for (unsigned int y = 0; y < 9; y++)
       
    {
       
      std::string line;
       
      std::cin >> line;
       
      for (unsigned int x = 0; x < 9; x++)
       
        board[x][y] = line[x] - '0';
       
    }
       
 
       
    // and replace all zeros (=Empty) with proper digits
       
    solve(board);
       
 
       
#ifdef ORIGINAL
       
    // the first the cells
       
    sum += 100 * board[0][0] + 10 * board[1][0] + board[2][0];
       
#else
       
    // print full solution
       
    for (unsigned int y = 0; y < 9; y++)
       
    {
       
      for (unsigned int x = 0; x < 9; x++)
       
        std::cout << board[x][y];
       
      std::cout << std::endl;
       
    }
       
#endif
       
  }
       
 
       
#ifdef ORIGINAL
       
  std::cout << sum;
       
#endif
       
 
       
  return 0;
       
}

This solution contains 13 empty lines, 15 comments and 12 preprocessor commands.