new test for four 0~9 numbers.
implement four 0~9 numbers to pass the test.
new test for four different numbers.
red, can't pass new test.
implement four different numbers to pass the test.
implement four different numbers to pass the test.
modifty to validate different level game input.
modify to generate answer with different levels.
make guess game with level.
illegal level flag will throw exception.
game is ok to play.
game is more interactive to play with more readable ouput.
refactor code.
add time class , refactor the code.