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This is a medium difficulty Coding & Algorithms question, commonly asked during Onsite rounds at Meta.

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This question is commonly asked for Software Engineer candidates at Meta during technical interviews.

Find shortest substring with N unique characters

Quick Overview

This question evaluates proficiency in string processing, handling character-uniqueness constraints, and algorithmic thinking about time and space efficiency.

Company: Meta

Role: Software Engineer

Category: Coding & Algorithms

Difficulty: medium

Interview Round: Onsite

You are given a string `s` and an integer `n`. Find the **shortest contiguous substring** of `s` that contains **exactly `n` distinct characters**. If there is no such substring, return an empty string (or `-1`, depending on how you choose to represent "not found"). If multiple substrings have the same minimal length, returning any one of them is acceptable. ### Requirements - Input: - `s`: a non-empty string composed of ASCII letters (you may assume lowercase English letters if you want to simplify). - `n`: an integer, `1 <= n <= 26` (assuming lowercase letters) and `n <= number of distinct characters in s` if a valid answer exists. - Output: - The shortest substring of `s` that contains exactly `n` distinct characters, or an indication that no such substring exists. ### Examples - Example 1: - `s = "aabcbc"`, `n = 2` - Valid substrings with exactly 2 distinct characters include: `"aa"`, `"ab"`, `"bc"`, `"cb"`, etc. - The shortest length is 2; any of `"aa"`, `"ab"`, `"bc"`, `"cb"` is acceptable. - Example 2: - `s = "abac"`, `n = 3` - Substrings with exactly 3 distinct characters: `"aba"` (a, b), `"abac"` (a, b, c), `"bac"` (b, a, c) - The shortest is `"bac"` of length 3. - Example 3: - `s = "aaaa"`, `n = 2` - There is no substring with exactly 2 distinct characters; return empty string or `-1`. ### Task 1. Specify the algorithm to solve this problem, including: - Time and space complexity. - How you handle edge cases such as `n = 0`, `n = 1`, `n > distinct characters in s`, and very long strings. 2. Then implement the algorithm in a programming language of your choice.

Quick Answer: This question evaluates proficiency in string processing, handling character-uniqueness constraints, and algorithmic thinking about time and space efficiency.

You are given a string `s` and an integer `n`. Find the **shortest contiguous substring** of `s` that contains **exactly `n` distinct characters**, and return it. If multiple substrings tie for the minimal length, return the **leftmost** one (the one that starts earliest). If no such substring exists, return the empty string `""`. ### Requirements - `s` is a string of ASCII letters (it may be empty). - `n` is an integer. If `n <= 0`, or `s` is empty, or there is no substring with exactly `n` distinct characters, return `""`. - A substring with **exactly** `n` distinct characters means the set of characters appearing in it has size exactly `n` (repeats of the same character do not add to the distinct count). ### Examples - `s = "aabcbc", n = 2` -> `"ab"` (length-2 windows with exactly 2 distinct: "ab", "bc", "cb"; leftmost is "ab". Note "aa" has only 1 distinct character.) - `s = "abac", n = 3` -> `"bac"` (shortest window with 3 distinct, length 3) - `s = "aaaa", n = 2` -> `""` (only 1 distinct character exists) - `s = "abcde", n = 1` -> `"a"` ### Task Implement an efficient sliding-window solution. State its time and space complexity and explain how you handle `n = 0`, `n = 1`, `n` greater than the number of distinct characters in `s`, and very long strings.

Constraints

0 <= len(s); s consists of ASCII letters (may be empty).
n may be any integer; n <= 0, empty s, or no valid window all return "".
If a valid answer exists, 1 <= n <= number of distinct characters in s.
On ties for minimal length, the leftmost (earliest-starting) substring is returned.

Examples

Input: ("aabcbc", 2)

Expected Output: "ab"

Explanation: Length-2 windows with exactly 2 distinct: "ab", "bc", "cb". Leftmost is "ab". ("aa" has only 1 distinct character.)

Input: ("abac", 3)

Expected Output: "bac"

Explanation: The shortest window containing 3 distinct characters {a,b,c} is "bac" at indices 1..3, length 3.

Hints

A window has 'exactly n distinct' which is harder than 'at most n'. Maintain a hash map of character counts and a running distinct-count for the current window [left, right].
Grow the window by moving right one step at a time. Whenever distinct exceeds n, shrink from the left. Also shrink while the leftmost character is redundant (count > 1) so the recorded window is as tight as possible.
Whenever distinct == n, the current window is a candidate; record it if it is strictly shorter than the best seen so far (strict comparison keeps the leftmost on ties). Handle n <= 0, empty s, and 'no valid window' by returning the empty string.

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