#0234

Longest Substring Without Repeating Characters

MediumDSA28 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 the cleanest sliding-window example for tracking a valid window and moving the left boundary only when needed.

Function Signature

def longest_unique_substring(s):

Examples

Example 1

Inputs = "abcabcbb"

Output3

The longest valid substrings include "abc", length 3.

Constraints

Return the exact requested value.
Handle empty or small inputs when the prompt allows them.
Prefer the target DSA pattern over brute force when the input can grow.

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

Repeated cycle

Input"abcabcbb"

Expected3

The best substring is length 3.

Left jump case

Input"abba"

Expected2

When the second b appears, left jumps past the first b.

Hidden Test Categories

empty stringall same characterall unique characters