Example 1
Input
s = 'abcabcbb'
Output
3
The longest substrings without repeating characters are 'abc' with length 3.
Full lesson preview
Find the longest substring without repeating characters
Find the length of the longest substring in a given string that contains no repeating characters using the sliding window technique.
Python practice15 minTwo Pointers & Sliding WindowIntermediateLast updated March 26, 2026
Problem statement
Given a string s, return the length of the longest substring that contains no repeating characters.
Use an efficient sliding-window approach that expands and shrinks the window while tracking seen characters to maintain uniqueness within the current window.
Task
Apply sliding window with character-index tracking to obtain an O(n) solution that finds the maximum-length substring without repeating characters.
Examples
Input format
Single argument: a string s.
Output format
Return an integer representing the length of the longest substring without repeating characters.
Constraints
0 <= len(s) <= 10^5. Characters can be any valid Python characters. Time target: O(n), extra space: O(min(n, alphabet)).
Samples
Sample 1
Input
s = 'pwwkew'
Output
3
The longest substring without repeating characters is 'wke' (or 'kew'), length 3.