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+# 問題へのリンク
+
+[3. Longest Substring Without Repeating Characters](https://leetcode.com/problems/longest-substring-without-repeating-characters/)
+
+# 言語
+Python
+
+# 問題の概要
+文字列 `s` が与えられたとき、重複しない文字からなる最長の部分文字列(substring)の長さを求める問題。
+
+# 自分の解法
+
+文字列から文字`char`を1文字ずつ取り出していく。文字`char`を走査しているとき、そこから前にたどって最長の部分文字列（で文字が重複していないもの）を管理しながら走査していく。
+
+- 各文字に対して、それが出現した最後のインデックスを記録するための辞書 `char_to_last_index` を用意してウィンドウ制御を行う。
+
+
+- 時間計算量：`O(n)`
+- 空間計算量：`O(1)`
+
+## step2
+- 変数名を変更：`substring_start_index` → `window_start_index`
+- コメントを追加
+
+
+# 別解・模範解答
+ウィンドウ制御にsetを用いる方法もある。
+
+https://wiki.python.org/moin/TimeComplexity より、setの要素の追加・削除は平均して`O(1)`であるため、以下のように書くことができる。
+
+```python
+while char in window_chars:
+    window_chars.remove(s[left_index])  # O(1)
+    left_index += 1
+```
+
+- Sliding Windowでは、基本的に`[left, right]`の閉区間で、`right`をforループでインクリメントしていくのが王道。バグも生みにくい。
+
+
+- 時間計算量：`O(n)`
+- 空間計算量：`O(1)`
+
+# 次に解く問題の予告
diff --git a/3_longest_substring_without_repeating_characters_medium/sliding_window_with_set.py b/3_longest_substring_without_repeating_characters_medium/sliding_window_with_set.py
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+#
+# @lc app=leetcode id=3 lang=python3
+#
+# [3] Longest Substring Without Repeating Characters
+#
+
+
+# @lc code=start
+class Solution:
+    def lengthOfLongestSubstring(self, s: str) -> int:
+        window_chars: set[str] = set()
+        max_length = 0
+        left_index = 0
+        for right_index, char in enumerate(s):
+            while char in window_chars:
+                window_chars.remove(s[left_index])  # O(1) on average
+                left_index += 1
+            window_chars.add(char)
+            max_length = max(max_length, right_index - left_index + 1)
+
+        return max_length
+
+
+# @lc code=end
diff --git a/3_longest_substring_without_repeating_characters_medium/step1.py b/3_longest_substring_without_repeating_characters_medium/step1.py
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+#
+# @lc app=leetcode id=3 lang=python3
+#
+# [3] Longest Substring Without Repeating Characters
+#
+
+
+# @lc code=start
+from collections import defaultdict
+
+
+class Solution:
+    def lengthOfLongestSubstring(self, s: str) -> int:
+        if not s:
+            return 0
+
+        # character -> last index
+        char_to_last_index: dict[str, int] = defaultdict(int)
+        substring_start_index = 0
+        max_length = 0
+
+        for index, char in enumerate(s):
+            if char in char_to_last_index:
+                substring_start_index = max(
+                    substring_start_index, char_to_last_index[char] + 1
+                )
+            char_to_last_index[char] = index
+            max_length = max(max_length, index - substring_start_index + 1)
+        return max_length
+
+
+# @lc code=end
diff --git a/3_longest_substring_without_repeating_characters_medium/step2.py b/3_longest_substring_without_repeating_characters_medium/step2.py
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+#
+# @lc app=leetcode id=3 lang=python3
+#
+# [3] Longest Substring Without Repeating Characters
+#
+
+
+# @lc code=start
+from collections import defaultdict
+
+
+class Solution:
+    def lengthOfLongestSubstring(self, s: str) -> int:
+        if not s:
+            return 0
+
+        # character -> last index
+        char_to_last_index: dict[str, int] = defaultdict(int)
+        window_start_index = 0
+        max_length = 0
+
+        # consider two substrings at every iteration;
+        # 1. trailing substring: starts at substring_start_index, ends at index, and managed by a window
+        # 2. last longest substring: only length information is saved as max_length
+        # at the end of every iteration, we update max_length
+        for index, char in enumerate(s):
+            if char in char_to_last_index:
+                window_start_index = max(
+                    window_start_index, char_to_last_index[char] + 1
+                )
+            char_to_last_index[char] = index
+            max_length = max(max_length, index - window_start_index + 1)
+        return max_length
+
+
+# @lc code=end
diff --git a/3_longest_substring_without_repeating_characters_medium/step3.py b/3_longest_substring_without_repeating_characters_medium/step3.py
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+#
+# @lc app=leetcode id=3 lang=python3
+#
+# [3] Longest Substring Without Repeating Characters
+#
+
+# @lc code=start
+from collections import defaultdict
+
+
+class Solution:
+    def lengthOfLongestSubstring(self, s: str) -> int:
+        # character -> last index
+        char_to_last_index: dict[str, int] = defaultdict(int)
+
+        # window manages the trailing substring
+        window_start_index = 0
+        max_length = 0
+        for index, char in enumerate(s):
+            if char in char_to_last_index:
+                window_start_index = max(
+                    window_start_index, char_to_last_index[char] + 1
+                )
+            max_length = max(max_length, index - window_start_index + 1)
+            char_to_last_index[char] = index
+        return max_length
+
+
+# @lc code=end