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+# 20. Valid Parentheses
+
+## STEP 1
+
+### 手作業でやってみる。
+
+* `{`, `[`, `(`, `}`, `]`, `)` と書いた紙が一列に並んでいる
+* 一番最初の紙から作業をしていく。
+* 紙が`{`, `[`, `(` だったら、机の上に縦に積んでいく。
+* 紙が`}`, `]`, `)` だったら、机の上の一番上から紙を取り、同じ種類の紙かを確認する。
+* 作業が完了して、机の上に何も残っていない場合には、終了する。
+* 机の上に何か残っている場合には、異常を報告する。
+
+```javascript
+var isValid = function(characters) {
+    const container = []
+    const open_bracket_chars = ["(", "{", "["]
+    for (const character of characters) {
+        if (open_bracket_chars.includes(character)) {
+            container.push(character)
+        }
+        if (character === ")") {
+            const last_character = container.pop()
+            if (last_character !== "(") {
+                return false
+            }
+        }
+        if (character === "]") {
+            const last_character = container.pop()
+            if (last_character !== "[") {
+                return false
+            }
+        }
+        if (character === "}") {
+            const last_character = container.pop()
+            if (last_character !== "{") {
+                return false
+            }
+        }
+    }
+    if (container.length === 0) {
+        return true
+    }
+    return false
+};
+```
+## STEP 2
+
+### やったこと
+* 関数を分けた.
+
+```javascript
+function doesMatchBracket(candidate, close_bracket) {
+    if (candidate === "[" && close_bracket === "]") {
+        return true
+    }
+    if (candidate === "{" && close_bracket === "}") {
+        return true
+    }
+    if (candidate === "(" && close_bracket === ")") {
+        return true
+    }
+
+    return false
+}
+var isValid = function(characters) {
+    const container = []
+    const open_bracket_chars = ["(", "{", "["]
+
+    for (const character of characters) {
+        // 開き括弧の場合
+        if (open_bracket_chars.includes(character)) {
+            container.push(character)
+            continue
+        }
+
+        // 閉じ括弧の場合
+        const open_bracket_candidate = container.pop()
+        const close_bracket = character
+        if (!doesMatchBracket(open_bracket_candidate, close_bracket)) {
+            return false
+        }
+    }
+    if (container.length === 0) {
+        return true
+    }
+
+    return false
+};
+```
+
+## STEP 3
+
+```javascript
+function doesMatchBracket(candidate, close_bracket_character) {
+    if (candidate === "(" && close_bracket_character === ")") {
+        return true
+    }
+    if (candidate === "[" && close_bracket_character === "]") {
+        return true
+    }
+    if (candidate === "{" && close_bracket_character === "}") {
+        return true
+    }
+    return false
+}
+var isValid = function(characters) {
+    const container =[]
+    const open_bracket_characters = ["(", "[", "{"]
+
+    for (const character of characters) {
+        if (open_bracket_characters.includes(character)) {
+            container.push(character)
+            continue
+        }
+        const open_bracket_candidate = container.pop()
+        const close_bracket_character = character
+        if (!doesMatchBracket(open_bracket_candidate, close_bracket_character)) {
+            return false
+        }
+    }
+
+    if (container.length === 0) {
+        return true
+    }
+    return false
+};
+```
+
+## 感想
+
+### 他の人のコードを読んで
+
+
+* BumbuShoji のPR https://github.com/BumbuShoji/Leetcode/pull/7
+  * 開き括弧と閉じ括弧の対応関係を表すMapを用意することも可能。 (`*1`で解法を追加)
+  
+  * はじめに、閉じ括弧があるケースを想定できていなかった。
+    * 配列が要素数0の時に、pop()で、undefinedを返すため、たまたま上手く行った。 
+      参考 : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/pop#return_value
+
+```javascript
+    // BEFORE
+    const open_bracket_candidate = container.pop()
+```
+
+```javascript
+    // AFTER
+    const open_bracket_candidate = container.pop() || ""
+    const open_bracket_candidate = container.length > 0 ? container.pop() : ""
+```
+
+  * 最後のif文は、`return container.length === 0`、`return !arr.length` でも良い
+
+```javascript
+  // BEFORE
+  if (container.length === 0) {
+      return true
+  }
+  return false
+```
+
+```javascript
+  // AFTER
+  return container.length === 0
+```
+
+* Odaのコメント https://discord.com/channels/1084280443945353267/1225849404037009609/1231646037077131315 
+  * 不正な入力に対して、エラーハンドリングをどうするかという視点がなかった。
+    https://github.com/lilnoahhh/leetcode/pull/7#discussion_r1948110757
+
+    > open_to_close でデータは持ちたいです。文字列にカッコ以外が来たときに落ちないで欲しいからです。
+
+     * 括弧以外の文字が入ってきた際のあるべきは、エラーログを吐き、exit 1で異常終了することと考えた。 https://discord.com/channels/1084280443945353267/1225849404037009609/1231648833914802267 
+     * 異常終了、続行する(例外を投げる、特殊な値を返す)の選択肢を意識して、選べると良い。
+       * 異常終了
+         * エラーに気づきやすい。 https://github.com/mura0086/arai60/pull/11#discussion_r1986104852
+         * 重要なエラー
+       * 続行
+         * 重要でないエラー
+       
+       参考 : https://discord.com/channels/1084280443945353267/1226508154833993788/1227171332131786774
+
+    * 異常な入力への対応を頭に入れてコードを書く。
+      参考 : https://discord.com/channels/1084280443945353267/1316770883729100810/1335077966954369095
+
+      * 質問されたら、結果と選択肢と理由を回答できる状態にする。 
+
+* lilnoahhhのPR https://github.com/lilnoahhh/leetcode/pull/7
+  * Stringで判定する方法がある。
+
+```javascript
+    // BEFORE
+    const open_bracket_chars = ["(", "{", "["]
+    if (open_bracket_chars.includes(character)) {
+      //
+    } 
+```
+
+```javascript
+    // AFTER
+    const open_brackets = "{(["
+    if (open_brackets.includes(character)) {
+      //
+    } 
+```
+
+
+* SanakoMeine のPR https://github.com/SanakoMeine/leetcode/pull/7 
+  * 簡潔で読みやすい.
+
+## その他の解法
+
+* `*1` 括弧の対応関係を表すMapを使う方法
+
+```javascript
+var isValid = function(characters) {
+    const open_to_close = new Map()
+    open_to_close.set("(", ")")
+    open_to_close.set("{", "}")
+    open_to_close.set("[", "]")
+
+    const container =[]
+    for (const character of characters) {
+        if (open_to_close.has(character)) {
+            container.push(character)
+            continue
+        }
+        const converted_last_character = open_to_close.get(container.pop())
+        const close_bracket_character = character
+        if (converted_last_character !== close_bracket_character) {
+            return false
+        }
+    }
+    return container.length === 0
+};
+```
+
+* `*2` 番兵をおく方法
+
+  * 以下の方法でさらに簡潔にかける.
+
+```javascript
+    const bracket_pairs = new Map([
+        ["{", "}"],
+        ["(", ")"],
+        ["[", "]"],
+        ["*", ""],
+    ])
+    const container = ["*"]
+```
+
+```javascript
+function doesMatchBracket(candidate, close_bracket_character) {
+    if (candidate === "(" && close_bracket_character === ")") {
+        return true
+    }
+    if (candidate === "[" && close_bracket_character === "]") {
+        return true
+    }
+    if (candidate === "{" && close_bracket_character === "}") {
+        return true
+    }
+    return false
+}
+var isValid = function(characters) {
+    const container =[]
+    const open_bracket_characters = ["(", "[", "{"]
+    container.push("SENTINEL")
+
+    for (const character of characters) {
+        if (open_bracket_characters.includes(character)) {
+            container.push(character)
+            continue
+        }
+        const open_bracket_candidate = container.pop()
+        const close_bracket_character = character
+        if (!doesMatchBracket(open_bracket_candidate, close_bracket_character)) {
+            return false
+        }
+    }
+    return container.length === 1 && container[container.length-1] === "SENTINEL"
+};
+```
+
+* `*3` 不正な入力のエラーハンドリングを行う解法
+  * 正常終了(エラーログ)と異常終了の選択肢のうち、異常終了を選択する。
+
+```javascript
+const isValid = function(characters) {
+    const open_to_close = new Map([
+        ["{", "}"],
+        ["[", "]"],
+        ["(", ")"]
+    ])
+    const container = []
+    const expected_characters = ["(", ")", "{", "}", "[", "]"]
+    for (const character of characters) {
+        if (!expected_characters.includes(character)) {
+            throw new Error(`Invalid character. Expected : ${expected_character.join(",")}. Current : ${character}`) // UPDATE
+        }
+        if (open_to_close.has(character)) {
+            container.push(character)
+            continue
+        }
+        const expected_close_bracket = open_to_close.get(container.pop())
+        const close_bracket_character = character
+        if (expected_close_bracket !== close_bracket_character) {
+            return false
+        }
+    }
+    return container.length === 0
+};
+```
+
+### レビューを受けて
+
+#### レビューコメント1
+
+* 変更前
+
+```javascript
+const isValid = function(characters) {
+    const open_to_close = new Map([
+        ["{", "}"],
+        ["[", "]"],
+        ["(", ")"]
+    ])
+    const container = []
+    const expected_characters = ["(", ")", "{", "}", "[", "]"]
+    for (const character of characters) {
+        if (open_to_close.has(character)) {
+            container.push(character)
+            continue
+        }
+        const expected_close_bracket = open_to_close.get(container.pop())
+        const close_bracket_character = character
+        if (expected_close_bracket !== close_bracket_character) {
+            return false
+        }
+    }
+    return container.length === 0
+};
+```
+
+* 変更後 (conatiner.pop()をする際にundefinedとstringの可能性を考慮する必要がある。)
+
+```javascript
+const isValid = function(characters) {
+    const open_to_close = new Map([
+        ["{", "}"],
+        ["[", "]"],
+        ["(", ")"]
+    ])
+    const container = []
+    const expected_characters = ["(", ")", "{", "}", "[", "]"]
+    for (const character of characters) {
+        if (open_to_close.has(character)) {
+            container.push(character)
+            continue
+        }
+        // 箱の中身が空で、閉じ括弧が挿入された際には、falseを返す。
+        if (container.length === 0) {
+            return false
+        }
+        const expected_close_bracket = open_to_close.get(container.pop())
+        const close_bracket_character = character
+        if (expected_close_bracket !== close_bracket_character) {
+            return false
+        }
+    }
+    return container.length === 0
+};