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>
       
 
       
// convert 3 digits to a 6 digit palindrome by mirroring and appending these 3 digits
       
// e.g. 234 becomes 234432

       
unsigned int makePalindrome(unsigned int x)
       
{
       
  unsigned int result = x * 1000;   // abc => abc000
       
  result +=    x / 100;             // a.. => a..00a
       
  result +=  ((x / 10) % 10) *  10; // .b. => .b.0b.
       
  result +=   (x % 10)       * 100; // ..c => ..cc..
       
  return result;
       
}
       
 
       
int main()
       
{
       
  unsigned int tests;
       
  std::cin >> tests;
       
  while (tests--)
       
  {
       
    // Hackerrank has a variable upper limit (instead of 1000000)
       
    unsigned int maximum;
       
    std::cin >> maximum;
       
 
       
    bool found = false;
       
    // find all palindromes, beginning with the largest
       
    // walk through all three-digit numbers
       
    for (auto upper3 = maximum / 1000; upper3 >= 100 && !found; upper3--)
       
    {
       
      // "mirror" these three digits to create a six-digit palindrome
       
      auto palindrome = makePalindrome(upper3);
       
      // too big ?
       
      if (palindrome >= maximum)
       
        continue;
       
 
       
      // split into two factors
       
      for (unsigned int i = 100; i * i <= palindrome; i++)
       
        if (palindrome % i == 0) // divisible ?
       
        {
       
          // make sure both factors must have three digits
       
          auto other = palindrome / i;
       
          if (other < 100 || other > 999)
       
            continue;
       
 
       
          std::cout << palindrome << std::endl;
       
          found = true;
       
          break;
       
        }
       
    }
       
  }
       
  return 0;
       
}

This solution contains 5 empty lines, 9 comments and 1 preprocessor command.