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 <string>
       
 
       
// return true if x matches the partial Collatz sequence

       
bool isGood(unsigned long long x, const std::string& sequence)
       
{
       
  for (auto s : sequence)
       
  {
       
    switch (x % 3)
       
    {
       
    case 0: // large downward step
       
      if (s != 'D')
       
        return false; // failed
       
 
       
      x /= 3;
       
      break;
       
 
       
    case 1: // upward step
       
      if (s != 'U')
       
        return false; // failed
       
 
       
      x = (4*x + 2) / 3;
       
      break;
       
 
       
    case 2: // small downward step
       
      if (s != 'd')
       
        return false; // failed
       
 
       
      x = (2*x - 1) / 3;
       
      break;
       
 
       
    default: ; // never reached
       
    }
       
  }
       
 
       
  return true;
       
}
       
 
       
int main()
       
{
       
  std::string sequence = "UDDDUdddDDUDDddDdDddDDUDDdUUDd";
       
  auto current = 1000000000000000ULL;
       
  std::cin >> current >> sequence;
       
 
       
  // initially search every number (3^0=1), then every third (3^1), every ninth (3^2), ...
       
  unsigned long long step = 1;
       
 
       
  // look for a match of the first characters of sequence
       
  for (size_t length = 1; length <= sequence.size(); length++)
       
  {
       
    // extract first characters
       
    auto partial = sequence.substr(0, length);
       
 
       
    // and find first match for those characters
       
    unsigned int iterations = 0;
       
    while (!isGood(current, partial))
       
    {
       
      current += step;
       
      if (++iterations > 100) // quick hack: prevent invalid input of live test
       
        return 1;
       
    }
       
 
       
    // increase step size for next iteration
       
    step *= 3;
       
  }
       
 
       
  // found it
       
  std::cout << current << std::endl;
       
  return 0;
       
}

This solution contains 14 empty lines, 7 comments and 2 preprocessor commands.