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Longest Substring Without Repeated Characters

CiscoSoftware Engineer · Technical Screen
EasySoftware EngineerTechnical ScreenCoding & Algorithms

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This problem tests string manipulation, hash map usage, and the sliding window technique to find a specific substring based on unique character constraints.

Problem Statement

Given a string s, find the leftmost longest contiguous substring that contains no repeating characters. If there are multiple substrings of the same maximum length, return the one that appears first in the input.

Function Signature

def longest_unique_substring(s: str) -> str:

Constraints

  • 0 <= len(s) <= 10^6
  • The string may contain any standard ASCII characters.
  • The solution should operate in O(n) time complexity, where n is the length of the string.

Examples

Example 1 Input: s = "abcabcbb" Output: "abc" Explanation: The longest substrings are "abc", "bca", and "cab", all of length 3. The leftmost one is "abc".

Example 2 Input: s = "bbbbb" Output: "b"

Example 3 Input: s = "" Output: ""

Edge Cases

  • Empty strings should return an empty string.
  • Strings with all unique characters should return the entire string.
  • Strings with all identical characters should return a single character.

Hints

Hint 1

Consider using a sliding window approach with two pointers, `left` and `right`, to maintain the current window of unique characters.

Hint 2

Use a hash map or a fixed-size array to store the last seen index of each character to quickly jump the `left` pointer when a duplicate is found.

Hint 3

Keep track of the start index and the maximum length found so far to extract the final substring only at the end.

Follow-up questions

  • How would your implementation change if the input were an infinitely long stream of characters instead of a static string?
  • How does the complexity change if the character set is Unicode/UTF-8 rather than standard ASCII?
  • What is the space complexity of your solution, and can it be further optimized if we know the input alphabet is limited to lowercase English letters?