Programming

3.longest-substring-without-repeating-characters

Description:

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

Constraints:

  • 0 <= s.length <= $5*10^4$
  • s consists of English letters, digits, symbols and spaces

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Appropriate Answer (Sliding Window):

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<?php

namespace LeetCode\LongestSubstringWithoutRepeatintCharacters;

class Solution
{
    /**
     * @param string $string
     * @return int
     */
    public function lengthOfLongestSubstring($string)
    {
        $length = strlen($string);
        if ($length == 0 || $length == 1) {
            return $length;
        }

        $answer = 0;
        $windowLeft = 0;
        $windowRight = 0;
        $usedCharacters = [];

        while ($windowRight < $length) {
            $character = $string[$windowRight];

            while (
                isset($usedCharacters[$character]) &&
                $usedCharacters[$character] >= $windowLeft
            ) {
                $windowLeft = $usedCharacters[$character] + 1;
            }

            $usedCharacters[$character] = $windowRight;
            $windowRight++;

            $answer = max($answer, $windowRight - $windowLeft);
        }

        return $answer;
    }
}
  • Time Complexity: $O(n)$

ʕ •ᴥ•ʔ：一開始有嘗試暴力解，但最後一個test case跑不過。

[2026-03-13] 更新
昨天解題遇到類似的概念 (sliding windows)
給 AI 寫 test case 沒過。

回來翻這篇文章，發現題目是一樣的，
把題目轉成 c# 寫法後，居然還是沒過！

驚覺原本的寫法有 bug (但 leetcode 988/988 cased passed)
最後修正成上面這個樣子。

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