From 593c0ddee9cbe4d6b49026dc460a8b632ebc03f7 Mon Sep 17 00:00:00 2001
From: Rytxxx <k.ryuto.5@icloud.com>
Date: Mon, 5 Jan 2026 00:54:22 -0800
Subject: [PATCH 1/2] 020-valid-parentheses

---
 020-valid-parentheses/memo.md  | 22 ++++++++++++++++++++++
 020-valid-parentheses/step1.py | 29 +++++++++++++++++++++++++++++
 020-valid-parentheses/step2.py |  8 ++++++++
 020-valid-parentheses/step3.py | 15 +++++++++++++++
 4 files changed, 74 insertions(+)
 create mode 100644 020-valid-parentheses/memo.md
 create mode 100644 020-valid-parentheses/step1.py
 create mode 100644 020-valid-parentheses/step2.py
 create mode 100644 020-valid-parentheses/step3.py

diff --git a/020-valid-parentheses/memo.md b/020-valid-parentheses/memo.md
new file mode 100644
index 0000000..d33e94a
--- /dev/null
+++ b/020-valid-parentheses/memo.md
@@ -0,0 +1,22 @@
+## step1で考えたこと
+
+それぞれの括弧に対応するものを取り除いていき、最後に残っていればFalse, そうでないならTrue
+
+=> whileの終了条件をどうするか？
+無限ループになってしまう
+さらにStep1のコードだと本来FalseであるべきものもTrueになってしまう
+
+カッコを分けて取り除くのではなく、セットで取り除けば良いのでは？？
+
+## step2
+コードは適切に求められたことを処理できているがRuntimeが若干遅いのが気になる
+Time Complexity: O(n^2). 
+Space Complexity: O(n). 
+=> Brute Force
+
+
+もっと早くするにはどうすべきか
+- whileを使わないで書く
+=> Using stack
+
+## step3
diff --git a/020-valid-parentheses/step1.py b/020-valid-parentheses/step1.py
new file mode 100644
index 0000000..4c4e947
--- /dev/null
+++ b/020-valid-parentheses/step1.py
@@ -0,0 +1,29 @@
+# 動かないコード
+
+class Solution:
+    def isValid(self, s: str) -> bool:
+        while s:
+            if "(" in s:
+                s.pop("(")
+                s.pop(")")
+            elif "[" in s:
+                s.pop("[")
+                s.pop("]")
+            elif "{" in s:
+                s.pop("{")
+                s.pop("}")
+        
+        return True
+
+class Solution:
+    def isValid(self, s: str) -> bool:
+        while s:
+            if "()" in s:
+                s = s.replace("()", "")
+            elif "[]" in s:
+                s = s.replace("[]", "")
+            elif "{}" in s:
+                s = s.replace("{}", "")
+        
+        if s: return True
+        else: return False
diff --git a/020-valid-parentheses/step2.py b/020-valid-parentheses/step2.py
new file mode 100644
index 0000000..cf40042
--- /dev/null
+++ b/020-valid-parentheses/step2.py
@@ -0,0 +1,8 @@
+# 以前見つけた回答を一度見てから自力で書いたコード
+class Solution:
+    def isValid(self, s: str) -> bool:
+        while "()" in s or "[]" in s or "{}" in s:
+            s = s.replace("()", "")
+            s = s.replace("[]", "")
+            s = s.replace("{}", "")
+        return s == ""
\ No newline at end of file
diff --git a/020-valid-parentheses/step3.py b/020-valid-parentheses/step3.py
new file mode 100644
index 0000000..b70e451
--- /dev/null
+++ b/020-valid-parentheses/step3.py
@@ -0,0 +1,15 @@
+## 別解
+class Solution:
+    def isValid(self, s: str) -> bool:
+        stack = []
+        closeToOpen = {")": "(", "]": "[", "}": "{"}
+
+        for c in s:
+            if c in closeToOpen:
+                if stack and stack[-1] == closeToOpen[c]:
+                    stack.pop()
+                else:
+                    return False
+            else:
+                stack.append(c)
+        return True if not stack else False
\ No newline at end of file

From baadf67b4d978c86efdefce19151c9f6868c8954 Mon Sep 17 00:00:00 2001
From: Rytxxx <k.ryuto.5@icloud.com>
Date: Tue, 6 Jan 2026 02:01:30 -0800
Subject: [PATCH 2/2] refactoring

---
 020-valid-parentheses/step1.py | 3 ++-
 020-valid-parentheses/step3.py | 6 +++---
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/020-valid-parentheses/step1.py b/020-valid-parentheses/step1.py
index 4c4e947..ef172e0 100644
--- a/020-valid-parentheses/step1.py
+++ b/020-valid-parentheses/step1.py
@@ -24,6 +24,7 @@ def isValid(self, s: str) -> bool:
                 s = s.replace("[]", "")
             elif "{}" in s:
                 s = s.replace("{}", "")
-        
+
+        # return not s と書くのが簡潔でわかりやすい。
         if s: return True
         else: return False
diff --git a/020-valid-parentheses/step3.py b/020-valid-parentheses/step3.py
index b70e451..5c275fa 100644
--- a/020-valid-parentheses/step3.py
+++ b/020-valid-parentheses/step3.py
@@ -2,11 +2,11 @@
 class Solution:
     def isValid(self, s: str) -> bool:
         stack = []
-        closeToOpen = {")": "(", "]": "[", "}": "{"}
+        open_to_close = {"()": ")", "[]": "]", "{": "}"}
 
         for c in s:
-            if c in closeToOpen:
-                if stack and stack[-1] == closeToOpen[c]:
+            if c in open_to_close:
+                if stack and stack[-1] == open_to_close[c]:
                     stack.pop()
                 else:
                     return False