Find shortest substring with n unique letters
Company: Microsoft
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: medium
Interview Round: Onsite
### Problem
You are given a string `s` consisting of lowercase English letters and an integer `n` (1 ≤ n ≤ 26).
Find the length of the shortest **contiguous** substring of `s` that contains **exactly** `n` distinct characters.
If no such substring exists, return `-1`.
#### Example
- Input: `s = "aabcbcdbca"`, `n = 3`
- One shortest substring with exactly 3 distinct characters is `"bca"` (length 3), so the output should be `3`.
Quick Answer: This question evaluates proficiency in string processing, frequency management, and algorithmic optimization for locating constrained contiguous substrings. Commonly asked in the coding & algorithms domain because it reveals how candidates reason about character distinctness and efficiency trade-offs, it targets practical application of algorithmic techniques rather than purely conceptual theory.