A string is defined as nice if, for every letter of the alphabet contained in the string, both its uppercase and lowercase forms are present.

Given a string s, return the longest substring of s that is nice. If there are multiple nice substrings of the maximum length, return the one with the earliest starting index. If no nice substring exists, return an empty string "".

Constraints:

• $1 \le s.length \le 100$

• s consists of uppercase and lowercase English letters.

Now, let’s take a moment to make sure you’ve correctly understood the problem. The quiz below helps you check if you’re solving the correct problem:

We have a game for you to play. Rearrange the logical building blocks to develop a clearer understanding of how to solve this problem.

Implement your solution in the following coding playground.