Longest Substring Without Repeating Characters

LeetCode Problem #3

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

Examples: Input: s = "abcabcbb" Output: 3
Explanation: The answer is "abc", with the length of 3.

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

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.

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

Hint: Generate all possible substrings, check each substring for duplicates, and update the maximum length accordingly.

Test Cases & Results

The following tests were used to verify the correctness of the algorithm:

• Standard LeetCode examples:

  • Input: 'abcabcbb' → Output: 3
  • Input: 'bbbbb' → Output: 1
  • Input: 'pwwkew' → Output: 3

• Digits:

  • Input: '1234567890' → Output: 10

• Symbols:

  • Input: '!@#$^&*()' → Output: 9

• Mixed letters and digits:

  • Input: 'a1b2c3d4' → Output: 8

• Mixed case sensitivity:

  • Input: 'AaBbCc' → Output: 6

• Whitespace:

  • Input: 'a a a a' → Output: 2

• Complex mixed characters:

  • Input: 'ab@12! cd#' → Output: 10

Notes:

• All results matched the expected outputs.
• Tests covered letters, digits, symbols, spaces, and mixed character sets to ensure full ASCII handling.

My quick reaction

Accepted ✅

• Runtime: 263 ms (~5%) — slow (O(N^2) on purpose this time).
• Memory: 120 MB (~6%) — heavy (extra buffers/allocs).

I’m not optimizing this one. I wanted correctness and to practice checking all substrings.
Next problems: I’ll push for more efficient solutions (O(N) patterns, less memory).

LeetCode final analysis (screenshots)