Question

How can I implement a function increment(string) that given string A produces string B where:

1. B > A (using Java’s compareTo logic)
2. There exists NO C where B > C > A

And the same for decrement(string) that given string A produces string B where:

1. B < A (using Java’s compareTo logic)
2. There exists NO C where B < C < A

EDIT

As pointed out in comments, this question is impossible to answer without some restrictions. The restriction is that you cannot append/remove new characters to/from the string unless it’s to handle under/over flow.

Answer 1

Increment is straightforward: B = A + "\u0000".

Decrement is impossible; the greatest string that is less than "Y", for example, is "X\uFFFF\uFFFF\uFFFF\uFFFF\uFFFF\uFFFF...".