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

Longest Substring Without Repeated Characters

Quick Overview

Return the leftmost longest contiguous substring whose characters are all unique. Handle empty input, ASCII and case-sensitive semantics, large strings, actual-substring output, streaming constraints, and the distinction among bytes, characters, and graphemes.

Company: Cisco

Role: Software Engineer

Category: Coding & Algorithms

Difficulty: easy

Interview Round: Technical Screen

## Problem Implement `longest_unique_substring(s)`. Return the longest contiguous substring of `s` that contains no repeated character. If several substrings have the same maximum length, return the leftmost one. Return the empty string when `s` is empty. ## Constraints - `0 <= len(s) <= 200,000` - `s` contains ASCII characters. - Character comparison is case-sensitive. ## Examples - `"abcabcbb"` returns `"abc"`. - `"bbbbb"` returns `"b"`. - `"pwwkew"` returns `"wke"`. - `""` returns `""`. ## Clarifications The answer must be an actual substring, not only its length. The tie rule is based on the smallest starting index. ## Hint Maintain a window with unique characters. When the next character repeats, move the left edge past the previous occurrence without moving it backward. ## Interview Follow-ups - Return the start index and length without copying the substring. - Adapt the method to a stream where old input cannot be reread. - Explain the difference between character, byte, and grapheme handling for non-ASCII text.

Quick Answer: Return the leftmost longest contiguous substring whose characters are all unique. Handle empty input, ASCII and case-sensitive semantics, large strings, actual-substring output, streaming constraints, and the distinction among bytes, characters, and graphemes.

Implement `longest_unique_substring(s)`. Return the longest contiguous substring of `s` whose characters are all distinct. If multiple maximum-length substrings exist, return the leftmost one, meaning the one with the smallest starting index. Return the empty string when `s` is empty. Return the substring itself rather than only its length.

Constraints

0 <= len(s) <= 200,000
s contains only ASCII characters.
Character comparison is case-sensitive.
When maximum-length substrings tie, return the one with the smallest starting index.

Examples

Input: ('',)

Expected Output: ''

Explanation: Empty input returns the empty string.

Input: ('z',)

Expected Output: 'z'

Explanation: A one-character input is already a unique substring.

Hints

Maintain a window with unique characters. When the next character repeats, move the left edge past the previous occurrence without moving it backward.

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