diff --git a/README.md b/README.md
index b8215fd..6693844 100644
--- a/README.md
+++ b/README.md
@@ -20,13 +20,14 @@ LeetCode の問題を以下手順で解く
 
 ## 解いた問題一覧
 
-| #   | Title                                                                                                     | Language | Difficulty |
-| --- | --------------------------------------------------------------------------------------------------------- | -------- | ---------- |
-| 5   | [Longest Palindromic Substring](https://leetcode.com/problems/longest-palindromic-substring/description/) | Python3  | Medium     |
-| 143 | [Reorder List](https://leetcode.com/problems/reorder-list/description/)                                   | Python3  | Medium     |
-| 207 | [Course Schedule](https://leetcode.com/problems/ourse-schedule/description/)                              | Python3  | Medium     |
-| 322 | [Coin Change](https://leetcode.com/problems/coin-change/description/)                                     | Python3  | Medium     |
-| 417 | [Pacific Atlantic Water Flow](https://leetcode.com/problems/pacific-atlantic-water-flow/description/)     | Python3  | Medium     |
+| #   | Title                                                                                                                                       | Language | Difficulty |
+| --- | ------------------------------------------------------------------------------------------------------------------------------------------- | -------- | ---------- |
+| 3   | [Longest Substring Without Repeating Characters](https://leetcode.com/problems/longest-substring-without-repeating-characters/description/) | Python3  | Medium     |
+| 5   | [Longest Palindromic Substring](https://leetcode.com/problems/longest-palindromic-substring/description/)                                   | Python3  | Medium     |
+| 143 | [Reorder List](https://leetcode.com/problems/reorder-list/description/)                                                                     | Python3  | Medium     |
+| 207 | [Course Schedule](https://leetcode.com/problems/ourse-schedule/description/)                                                                | Python3  | Medium     |
+| 322 | [Coin Change](https://leetcode.com/problems/coin-change/description/)                                                                       | Python3  | Medium     |
+| 417 | [Pacific Atlantic Water Flow](https://leetcode.com/problems/pacific-atlantic-water-flow/description/)                                       | Python3  | Medium     |
 
 ## その他
 
diff --git a/problems/medium/python3/3/longest-substring-without-repeating-characters.step1.py b/problems/medium/python3/3/longest-substring-without-repeating-characters.step1.py
new file mode 100644
index 0000000..df1e014
--- /dev/null
+++ b/problems/medium/python3/3/longest-substring-without-repeating-characters.step1.py
@@ -0,0 +1,19 @@
+#
+# @lc app=leetcode id=3 lang=python3
+#
+# [3] Longest Substring Without Repeating Characters
+#
+
+# @lc code=start
+class Solution:
+    def lengthOfLongestSubstring(self, s: str) -> int:
+        index_map = dict()
+        start = -1
+        length = 0
+        for i, char in enumerate(s):
+            if char in index_map:
+                start = max(start, index_map[char])
+            index_map[char] = i
+            length = max(length, i - start)
+        return length
+# @lc code=end
diff --git a/problems/medium/python3/3/longest-substring-without-repeating-characters.step2.py b/problems/medium/python3/3/longest-substring-without-repeating-characters.step2.py
new file mode 100644
index 0000000..89c3c23
--- /dev/null
+++ b/problems/medium/python3/3/longest-substring-without-repeating-characters.step2.py
@@ -0,0 +1,19 @@
+#
+# @lc app=leetcode id=3 lang=python3
+#
+# [3] Longest Substring Without Repeating Characters
+#
+
+# @lc code=start
+class Solution:
+    def lengthOfLongestSubstring(self, s: str) -> int:
+        char_index_map = dict()
+        start = -1
+        longest_length = 0
+        for i, char in enumerate(s):
+            if char in char_index_map:
+                start = max(start, char_index_map[char])
+            char_index_map[char] = i
+            longest_length = max(longest_length, i - start)
+        return longest_length
+# @lc code=end
diff --git a/problems/medium/python3/3/longest-substring-without-repeating-characters.step3.py b/problems/medium/python3/3/longest-substring-without-repeating-characters.step3.py
new file mode 100644
index 0000000..c41c476
--- /dev/null
+++ b/problems/medium/python3/3/longest-substring-without-repeating-characters.step3.py
@@ -0,0 +1,19 @@
+#
+# @lc app=leetcode id=3 lang=python3
+#
+# [3] Longest Substring Without Repeating Characters
+#
+
+# @lc code=start
+class Solution:
+    def lengthOfLongestSubstring(self, s: str) -> int:
+        char_to_index = dict()
+        start = 0
+        longest_length = 0
+        for i, char in enumerate(s):
+            if char in char_to_index:
+                start = max(start, char_to_index[char] + 1)
+            char_to_index[char] = i
+            longest_length = max(longest_length, i - start + 1)
+        return longest_length
+# @lc code=end