Given two String inputs a and b, find and return the three-character substring from a that occurs the most times in b. If no substrings from a are found in b, return an empty String. If a tie occurs, return the first substring from a that occurs the most times. Precondition: a.length() > 2.

HINT: you will probably want a nested loop structure here as well, but the two loops are likely to have very different bounds.