Given a string s and an integer k, return the number of substrings of s of length k that contain only unique characters.

Constraints:

• $1 \leq$ s.length $\leq 10^4$

• s consists of lowercase English letters.

• $1 \leq$ k $\leq 10^4$

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.