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>
       
 
       
// return true if root^2 matches 1_2_3_4_5_6_7_8_9_0

       
bool match(unsigned long long x)
       
{
       
  unsigned long long square = x * x;
       
 
       
  // required digits in reverse order
       
  const unsigned int Right2Left[] = { 0, 9, 8, 7, 6, 5, 4, 3, 2, 1 };
       
  unsigned int index = 0;
       
 
       
  // check all digits
       
  do
       
  {
       
    // right-most digits matches current element of Right2Left ?
       
    auto digit = square % 10;
       
    if (digit != Right2Left[index++])
       
      return false;
       
 
       
    // remove the digit which passes the test and skip the next digit which is unknown, too
       
    square /= 100;
       
  } while (square > 0);
       
 
       
  // all tests passed !
       
  return true;
       
}
       
 
       
int main()
       
{
       
  // smallest possible number: gaps are zeros => sqrt(1020304050607080900)
       
  const unsigned int MinNumber = 1010101010;
       
  // largest  possible number: gaps are nines => sqrt(1929394959697989990)
       
  const unsigned int MaxNumber = 1389026620;
       
 
       
  //for (unsigned int x = MinNumber; x <= MaxNumber; x += 10)
       
  for (unsigned int x = MaxNumber; x >= MinNumber; x -= 10)
       
    if (match(x))
       
    {
       
      std::cout << x << std::endl;
       
      break;
       
    }
       
 
       
  return 0;
       
}

This solution contains 8 empty lines, 9 comments and 1 preprocessor command.