Leetcode - 3. Longest Substring Without Repeating Characters

Medium

Dec 22, 2025

#sliding-window

3. Longest Substring Without Repeating Characters

Given a string s, find the length of the longest substring without duplicate characters.

Example 1:

Input: s = "abcabcbb"
Output: 3
Explanation: The answer is "abc", with the length of 3. Note that "bca" and "cab" are also correct answers.

Example 2:

Input: s = "bbbbb"
Output: 1
Explanation: The answer is "b", with the length of 1.

Example 3:

Input: s = "pwwkew"
Output: 3
Explanation: The answer is "wke", with the length of 3.
Notice that the answer must be a substring, "pwke" is a subsequence and not a substring.

0 <= s.length <= 5 * 104
s consists of English letters, digits, symbols and spaces.

Constraints:

Notes

Intuition

A sliding window maintains the current substring. When a duplicate character enters from the right, shrink from the left until the duplicate is removed, ensuring all characters in the window are unique.

Implementation

Use a set to track characters in the current window. Iterate with the right pointer. If the character at r is already in the set, remove characters from the left (incrementing l) until the duplicate is gone. Add the current character to the set and update the maximum length (r - l + 1).

Edge-cases

The window size calculation is r - l + 1 (inclusive on both ends). Empty strings return 0 naturally since the loop doesn't execute.

Time: O(n) — each character added and removed from set at most once
Space: O(min(n, m)) — where m is the character set size (26 for lowercase letters)

Complexity

Solution

from typing import List

class Solution:
    def lengthOfLongestSubstring(self, s: str) -> int:
        max_length = 0
        charSet = set()
        l = 0
        for r in range(len(s)):
            while s[r] in charSet:
                charSet.remove(s[l])
                l += 1
            charSet.add(s[r])
            max_length = max(max_length, r - l + 1)

        return max_length
        



if __name__ == "__main__":
    # Include one-off tests here or debugging logic that can be run by running this file
    # e.g. print(solution.two_sum([1, 2, 3, 4], 3))
    solution = Solution()