diff --git a/arai60/valid-parentheses/README.md b/arai60/valid-parentheses/README.md
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+## 考察
+- 過去に解いたことあり
+- 方針
+    - stackを使って対応を見ていく
+        - 前から文字を見ていって、open bracketが来たらstackに積む
+        - close bracketが来たら、stackから要素をpopして対応をチェックする(stackが空じゃないかだけ注意)
+        - 最後にstackが空になっていたらOK
+        - time: O(n), space: O(n)
+- あとは実装
+
+## Step1
+- 上のアルゴリズムを実装
+- time: O(n), space: O(n)
+
+## Step2
+- 手続き的にbracketの対応関係を判定していた箇所を、辞書を使って宣言的な記述に変更
+    - 結果、余計なif文がかなり減った
+
+## Step3
+- 1回目: 1m51s
+- 2回目: 1m53s
+- 3回目: 1m28s
+
+## Step4
+- レビューを元にコードを修正
+- 変数名の見直し
+    - stack -> bracket_stack
+        - 何を入れるstackなのかを明確に
+    - bracket_pairs -> open_to_close
+        - 例えば、open_to_close[open_bracket] という記述を見たときにclose_bracketが返ってくることが一目でわかる
+        - より具体的になって、わかりやすくなった感覚
+- `if not open_to_close[open_bracket] == c:` と書いていたところを `if open_to_close[open_bracket] != c:`に
+- early continueしてネストを浅く
diff --git a/arai60/valid-parentheses/step1.py b/arai60/valid-parentheses/step1.py
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+class Solution:
+    def isValid(self, s: str) -> bool:
+        stack = []
+        for c in s:
+            if c in ['(', '{', '[']:
+                stack.append(c)
+            else:
+                if not stack:
+                    return False
+
+                open_bracket = stack.pop()
+                if c == ')':
+                    if not open_bracket == '(':
+                        return False
+                elif c == '}':
+                    if not open_bracket == '{':
+                        return False
+                else:
+                    if not open_bracket == '[':
+                        return False
+
+        return not stack
diff --git a/arai60/valid-parentheses/step2.py b/arai60/valid-parentheses/step2.py
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+class Solution:
+    def isValid(self, s: str) -> bool:
+        stack = []
+        bracket_pairs = {
+            '(': ')',
+            '{': '}',
+            '[': ']',
+        }
+        for c in s:
+            if c in bracket_pairs:
+                stack.append(c)
+            else:
+                if not stack:
+                    return False
+
+                open_bracket = stack.pop()
+                if not bracket_pairs[open_bracket] == c:
+                    return False
+
+        return not stack
diff --git a/arai60/valid-parentheses/step3.py b/arai60/valid-parentheses/step3.py
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+class Solution:
+    def isValid(self, s: str) -> bool:
+        stack = []
+        bracket_pairs = {
+            '(': ')',
+            '{': '}',
+            '[': ']',
+        }
+        for c in s:
+            if c in bracket_pairs:
+                stack.append(c)
+            else:
+                if not stack:
+                    return False
+
+                open_bracket = stack.pop()
+                if not bracket_pairs[open_bracket] == c:
+                    return False
+
+        return not stack
diff --git a/arai60/valid-parentheses/step4.py b/arai60/valid-parentheses/step4.py
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+++ b/arai60/valid-parentheses/step4.py
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+class Solution:
+    def isValid(self, s: str) -> bool:
+        bracket_stack = []
+        open_to_close = {
+            '(': ')',
+            '{': '}',
+            '[': ']',
+        }
+        for c in s:
+            if c in open_to_close:
+                bracket_stack.append(c)
+                continue
+
+            if not bracket_stack:
+                return False
+
+            open_bracket = bracket_stack.pop()
+            if open_to_close[open_bracket] != c:
+                return False
+
+        return not bracket_stack