#0054

Longest Substring Without Repeating Characters

MediumDSA25 min20 XP

Problem

Given a string s, return the length of the longest contiguous substring that contains no repeated characters.

Why This Matters

This is one of the cleanest tests of whether you understand why the left pointer can safely jump forward instead of restarting the scan.

Function Signature

def longest_unique_substring(s):

Examples

Example 1

Inputs = "abcabcbb"

Output3

The longest valid substrings include "abc", "bca", and "cab".

Constraints

Return 0 for an empty string.
Treat uppercase and lowercase as different characters.
Aim for O(n) time.

CodePython

def longest_unique_substring(s):
    """
    Input example: s = "abcabcbb"
    Return: 3
    Return the value from the function so the tests can compare it. Do not print unless the prompt asks.
    """
    # write your code here
    return 0

def longest_unique_substring(s):
    """
    Input example: s = "abcabcbb"
    Return: 3
    Return the value from the function so the tests can compare it. Do not print unless the prompt asks.
    """
    # write your code here
    return 0

Visible browser tests run here when available.

Testcases2 visible / 3 hidden categories

Classic repeat pattern

Input"abcabcbb"

Expected3

The best window length is 3.

Repeated same character

Input"bbbbb"

Expected1

Only one repeated character can stay in the window.

Hidden Test Categories

Empty stringRepeat appears before current left pointerMixed case characters