8 string to integer atoi medium

8_string_to_integer_atoi_medium/README.md

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+# 問題へのリンク
+[8. String to Integer (atoi)](https://leetcode.com/problems/string-to-integer-atoi/)
+
+# 言語
+Python
+
+
+# 自分の解法
+dislikesが多いのは、LeetCodeらしからぬ細かい仕様が多いから？
+## step1
+
+```python
+class Solution:
+    CHAR_TO_DIGIT = {str(i): i for i in range(10)}
+
+    def myAtoi(self, s: str) -> int:
+        SPACE = " "
+        PLUS = "+"
+        MINUS = "-"
+        MIN = -(2**31)
+        MAX = 2**31 - 1
+        if not s:
+            return 0
+        num = 0
+        # remove leading whitespaces
+        i = 0
+        while i < len(s) and s[i] == SPACE:
+            i += 1
+        s = s[i:]
+        if not s:
+            return 0
+        # process sign
+        is_negative = False
+        if s[0] == PLUS:
+            s = s[1:]
+        elif s[0] == MINUS:
+            s = s[1:]
+            is_negative = True
+        if not s:
+            return 0
+        # conversion
+        digits: list[int] = []
+        for char in s:
+            if char not in self.CHAR_TO_DIGIT:
+                break
+            digit = self.CHAR_TO_DIGIT[char]
+            digits.append(digit)
+        digits.reverse()
+        for i, digit in enumerate(digits):
+            num += digit * pow(10, i)
+        num = -num if is_negative else num
+        # rounding
+        num = min(num, MAX)
+        num = max(num, MIN)
+
+        return num
+```
+- leading whitespace を取り除く箇所で何度かミスってしまった
+    - まず`s.replace(" ", "")`で全ての空白を取り除いてしまっていた
+    - 次に前から空白を取り除く処理を実装したが、何度かバグを踏んだ
+    - `while i < len(s) and ...`と先にandしておくと、`i`が`len(s)`に達したときに`s[i]`でIndexErrorになる問題を防げる
+
+- 時間計算量：`O(len(s))`
+- 空間計算量：`O(1)`
+
+Tests
+- standard cases:
+    - `s="  00321a3"` -> 321
+    - `s="  +00321a3"` -> 321
+    - `s="  -00321a3"` -> -321
+- empty:
+    - `s=""` -> 0
+- invalid cases:
+    - `s=" "` -> 0
+    - `s="a"` -> 0
+    - `s="a1"` -> 0
+- whitespaces:
+    - `s="  1"` -> 1
+- signedness:
+    - `s="1"` -> 1
+    - `s="+1"` -> 1
+    - `s="-1"` -> -1
+- leading zeros:
+    - `s="0010"` -> 10
+- rounded cases:
+    - `s="-1_000_000_000_000"` -> -2**31
+    - `s="1_000_000_000_000"` -> 2**31-1
+
+## step2
+
+```python
+class Solution:
+    def myAtoi(self, s: str) -> int:
+        SPACE = " "
+        PLUS = "+"
+        MINUS = "-"
+        MIN_INT = -(2**31)
+        MAX_INT = 2**31 - 1
+
+        if not s:
+            return 0
+        num = 0
+
+        # remove leading whitespaces
+        index = 0
+        while index < len(s) and s[index] == SPACE:
+            index += 1
+        # process sign
+        sign = 1
+        if index < len(s) and s[index] == PLUS:
+            index += 1
+        elif index < len(s) and s[index] == MINUS:
+            sign = -1
+            index += 1
+        # conversion
+        while index < len(s) and "0"<=s[index]<="9":
+            digit = ord(s[index]) - ord("0")
+            num = num * 10 + digit
+            index += 1
+
+        num = sign * num
+        # rounding
+        num = min(num, MAX_INT)
+        num = max(num, MIN_INT)
+
+        return num
+```
+
+
+- 文字列のスライスを使わないようにした。これにより、`O(n)`の時間計算量と`O(1)`の空間計算量で実装できる
+- `is_negative`を`sign`に変更した
+    - `sign`を`1`か`-1`にしておくと、最後に`num=sign*num`とするだけで済む
+
+
+最後の変換部分のロジックを
+```python
+digits.reverse()
+for i, digit in enumerate(digits):
+    num += digit * pow(10, i)
+```
+から
+```python
+for digit in digits:
+    num = num * 10 + digit
+```
+と書き換えた
+- `digits`を逆順にする必要がなくなった
+- `pow`を使わなくなった
+- `num=0`から始めて、`num=num*10+digit`を繰り返すことで、上の位の桁から順に処理できる
+
+- > `ord(s[pos]) - ord('0')`の代わりに`int(s[pos])`とかも選択肢としてあり。とはいえこれをするんだったら`int(s[index:])`みたいにやっていいじゃんという気持ちにもなるので、この問題的にはint使わない方が空気が読めてそう。
+- `ord`を使う方法もある
+(ref: https://github.com/hayashi-ay/leetcode/pull/69/files)
+    - Pythonでは文字列にも比較演算子が使えるので、`"0" <= s[i] <= "9"`のようにするのが良いかも
+- `is_digit`メソッドはASCIIコード以外にも対応したメソッドなので、今回は使わなくて良い
+
+## step3
+```python
+class Solution:
+    def myAtoi(self, s: str) -> int:
+        if not s:
+            return 0
+        index = 0
+        # Process whitespace
+        SPACE = " "
+        while index < len(s) and s[index] == SPACE:
+            index += 1
+
+        # Process signedness
+        is_negative = False
+        MINUS = "-"
+        PLUS = "+"
+        if index < len(s) and s[index] == MINUS:
+            is_negative = True
+            index += 1
+        elif index < len(s) and s[index] == PLUS:
+            index += 1
+
+        # Process conversion
+        num = 0
+        # leading zeros
+        while index < len(s) and s[index] == str(0):
+            index += 1
+        digits = [str(i) for i in range(10)]
+        while index < len(s) and s[index] in digits:
+            digit = int(s[index])
+            num = num*10 + digit
+            index += 1
+
+        # apply signedness
+
+        num = -num if is_negative else num
+
+        # Process rounding
+        INT_MIN = -2**31
+        INT_MAX = 2**31 -1
+        num = min(num, INT_MAX)
+        num = max(num, INT_MIN)
+        return num
+```
+
+- `ord`や、文字列の比較を忘れていた
+- leading zero の処理を別途書いたが、これはなくても良かった
+
+## step4 (FB)
+
+
+## その他
+
+# 次に解く問題の予告
+- [Number of Islands](https://leetcode.com/problems/number-of-islands/description/)

8_string_to_integer_atoi_medium/step1.py

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+#
+# @lc app=leetcode id=8 lang=python3
+#
+# [8] String to Integer (atoi)
+#
+
+# @lc code=start
+class Solution:
+    CHAR_TO_DIGIT = {str(i): i for i in range(10)}
+
+    def myAtoi(self, s: str) -> int:
+        SPACE = " "
+        PLUS = "+"
+        MINUS = "-"
+        MIN = -(2**31)
+        MAX = 2**31 - 1
+        if not s:
+            return 0
+        num = 0
+        # remove leading whitespaces
+        i = 0
+        while i < len(s) and s[i] == SPACE:
+            i += 1
+        s = s[i:]
+        if not s:
+            return 0
+        # process sign
+        is_negative = False
+        if s[0] == PLUS:
+            s = s[1:]
+        elif s[0] == MINUS:
+            s = s[1:]
+            is_negative = True
+        if not s:
+            return 0
+        # conversion
+        digits: list[int] = []
+        for char in s:
+            if char not in self.CHAR_TO_DIGIT:
+                break
+            digit = self.CHAR_TO_DIGIT[char]
+            digits.append(digit)
+        digits.reverse()
+        for i, digit in enumerate(digits):
+            num += digit * pow(10, i)
+        num = -num if is_negative else num
+        # rounding
+        num = min(num, MAX)
+        num = max(num, MIN)
+
+        return num
+
+
+# @lc code=end

8_string_to_integer_atoi_medium/step2.py

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+#
+# @lc app=leetcode id=8 lang=python3
+#
+# [8] String to Integer (atoi)
+#
+
+# @lc code=start
+class Solution:
+    def myAtoi(self, s: str) -> int:
+        SPACE = " "
+        PLUS = "+"
+        MINUS = "-"
+        MIN_INT = -(2**31)
+        MAX_INT = 2**31 - 1
+
+        if not s:
+            return 0
+        num = 0
+
+        # remove leading whitespaces
+        index = 0
+        while index < len(s) and s[index] == SPACE:
+            index += 1
+        # process sign
+        sign = 1
+        if index < len(s) and s[index] == PLUS:
+            index += 1
+        elif index < len(s) and s[index] == MINUS:
+            sign = -1
+            index += 1
+        # conversion
+        while index < len(s) and "0"<=s[index]<="9":
+            digit = ord(s[index]) - ord("0")
+            num = num * 10 + digit
+            index += 1
+
+        num = sign * num
+        # rounding
+        num = min(num, MAX_INT)
+        num = max(num, MIN_INT)
+
+        return num
+
+
+# @lc code=end

8_string_to_integer_atoi_medium/step3.py

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+#
+# @lc app=leetcode id=8 lang=python3
+#
+# [8] String to Integer (atoi)
+#
+
+# @lc code=start
+class Solution:
+    def myAtoi(self, s: str) -> int:
+        if not s:
+            return 0
+        index = 0
+        # Process whitespace
+        SPACE = " "
+        while index < len(s) and s[index] == SPACE:
+            index += 1
+
+        # Process signedness
+        is_negative = False
+        MINUS = "-"
+        PLUS = "+"
+        if index < len(s) and s[index] == MINUS:
+            is_negative = True
+            index += 1
+        elif index < len(s) and s[index] == PLUS:
+            index += 1
+
+        # Process conversion
+        num = 0
+        # leading zeros
+        while index < len(s) and s[index] == str(0):
+            index += 1
+        digits = [str(i) for i in range(10)]
+        while index < len(s) and s[index] in digits:
+            digit = int(s[index])
+            num = num*10 + digit
+            index += 1
+
+        # apply signedness
+
+        num = -num if is_negative else num
+
+        # Process rounding
+        INT_MIN = -2**31
+        INT_MAX = 2**31 -1
+        num = min(num, INT_MAX)
+        num = max(num, INT_MIN)
+        return num
+
+
+# @lc code=end