You are given a string num representing a large integer. An integer is considered good if it satisfies the following conditions:

• It is a substring of num of length $3$.

• It consists of only one unique digit.

Return the maximum good integer as a string. If no such integer exists, return an empty string "".

Note: A substring is a contiguous sequence of characters within a string. If multiple good integers exist, compare their numeric values to find the maximum.

Constraints:

• $3 \leq$ num.length $\leq 1000$

• num consists only of digits.

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.