Basic example
Input
s = "abcabcbb"
Output
3
The longest substring without repeating characters is "abc", with length 3.
Full lesson preview
Find the length of the longest substring without repeating characters
Compute the length of the longest substring with all unique characters using a sliding window and index map.
Python practice14 minStrings – Sliding Window & FrequencyIntermediateLast updated March 25, 2026
Problem statement
Given a string s, return the length of the longest substring that contains no repeating characters. Use a sliding window and track characters' last seen positions to adjust the left boundary efficiently.
Task
Use a dynamic sliding window and a map of last-seen indices to maintain a window of unique characters and compute the maximum length.
Examples
Input format
One string argument: s. Example: length_of_longest_substring("pwwkew")
Output format
An integer representing the length of the longest substring without repeating characters.
Constraints
0 <= len(s) <= 10^5. Characters can be any valid string characters. Aim for O(n) time and O(min(n, alphabet)) space.
Samples
Sample 1
Input
s = "bbbbb"
Output
1
The longest substring without repeating characters is "b".