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.

       
#include <iostream>
       
#include <vector>
       
 
       
int main()
       
{
       
  const unsigned int limit = 50000000;
       
  // all bits set to zero
       
  std::vector<bool> atLeastOne (limit, false);
       
  std::vector<bool> moreThanOne(limit, false);
       
 
       
  for (unsigned int a = 1; a < limit; a++)
       
    for (auto b = (a + 3) / 4; b < a; b++)
       
    {
       
      auto current = a * (4*b - a);
       
      if (current >= limit)
       
        break;
       
 
       
      // already had a solution (or more) ?
       
      if (atLeastOne[current])
       
        moreThanOne[current] = true;
       
      else
       
        // nope, first solution
       
        atLeastOne [current] = true;
       
    }
       
 
       
  // count all with exactly 1 solution
       
  unsigned int count = 0;
       
  for (unsigned int i = 0; i < limit; i++)
       
    if (atLeastOne[i] && !moreThanOne[i])
       
      count++;
       
 
       
  std::cout << count << std::endl;
       
 
       
  return 0;
       
}

This solution contains 6 empty lines, 4 comments and 2 preprocessor commands.