Longest Substring Without Repeating Characters

Find the length of the longest substring without repeating characters using an efficient sliding window.

strings sliding-window two-pointers hash-map

Write a function **`length_of_longest_substring(s: str) -> int`** that takes a string `s` and returns the length of the longest substring without repeating characters. A substring is a contiguous sequence of characters within the string. For example, in `"abcabcbb"`, the longest substring without repeating characters is `"abc"` (length 3). Your solution must run in O(n) time and O(min(n, alphabet_size)) space.

Constraints

Input string length: 0 ≤ len(s) ≤ 10^5. Characters are any printable ASCII characters (32–126). The solution should be efficient enough for the maximum length.

Example

>>> length_of_longest_substring("abcabcbb")
3
>>> length_of_longest_substring("bbbbb")
1
>>> length_of_longest_substring("pwwkew")
3
>>> length_of_longest_substring("")
0

10 points ~15 min

Recent Submissions

No submissions yet — hit Run Tests to try!

Hints

Use two pointers (left and right) to maintain a valid window of unique characters.

Store the last seen index of each character in a dictionary to move the left pointer efficiently.

When a repeat is found, update the left pointer to one past the previous occurrence, then continue expanding the right pointer.

Track the maximum window size encountered at each step.

Test Results

Press Ctrl+Enter or click Run Tests to execute your code.