Exercise: run-length encoding

Write a method static String compress(String s) that replaces each run of identical characters in a row with the character followed by the run's length.

• compress("aaabcc") → "a3b1c2"
• compress("abc") → "a1b1c1"
• compress("") → ""

Constraints: an uppercase and a lowercase letter are different characters; a run may be longer than 9 characters, and then its length takes several digits; a character that appears in several places gives several runs; an empty string gives an empty string.

Do not add extra imports unless the task says so explicitly.