Guess the number which contain 4 non-repetitive digits by using xAxB tips.
For example:
The answer number is 0921
If user guess 0123, it would reply 1A2B. Means 1 digit in correct position ("0") and 2 digits in wrong position ("1" and "2").
If user guess 0923, it would reply 3A0B. Means 3 digits in correct position ("0", "9" and "2").
So, while the 4A0B replied, means you win~