Number Slide Puzzle

This is the classic number slide puzzle. You start with a shuffled board of numbers and you must slide them into numerical order from 1 to 15. NOTE: Often the game cannot be solved - this even occurs in physical versions of this game you can play with your hands. I don't know how or why it can't be solved sometimes, but it's always at the end with 12 and 11 ending up in the reversed spots. I don't know what to do about it.