Create 8. String to Integer (atoi).md

競技プロ就活部PR用/8. String to Integer (atoi).md

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+解けなかったため、他参加者の答えを参考に何度か書いてみた後、自力で再現。
+### 1回目
+時間計算量: O(N)<br>
+空間計算量: O(N)<br>
+N: len(s)
+
+```python
+class Solution:
+    def myAtoi(self, s: str) -> int:
+        s = s.lstrip()
+        if not s:
+            return 0
+
+        negative_flag = False
+        index = 0
+        if s[index] == "-" or s[index] == "+":
+            if s[index] == "-":
+                negative_flag = True
+            index += 1
+
+        digits_str = ""
+        while index < len(s):
+            if s[index].isdigit():
+                digits_str += s[index]
+                index += 1
+            else:
+                break
+        if not digits_str:
+            return 0
+
+        digits_int = int(digits_str)
+        if negative_flag:
+            digits_int *= -1
+
+        MAX_INT = 2 ** 31 - 1
+        MIN_INT = -2 ** 31
+        if digits_int > MAX_INT:
+            return MAX_INT
+        elif digits_int < MIN_INT:
+            return MIN_INT
+        else:
+            return digits_int
+```
+
+### 2回目
+他参加者の回答を参考に、下記を書き換えてみる。
+- lstripを自前で実装。
+- negative_flagをsignに変更。
+- 符号の条件判定を、in ('-', '+')に変更。
+- INT_MAX, INT_MINをビット演算で実装。
+- intへの変換を自前で実装。(ordの利用)
+- 実装後、各種フラッグなどの変数を上に移動。
+
+sを直接いじるより、indexを動かしていく方がわかりやすくかつsを壊さないため安心かと思った。
+
+```python
+class Solution:
+    def myAtoi(self, s: str) -> int:
+        index = 0
+        sign = 1
+        MAX_INT = (1 << 31) - 1
+        MIN_INT = - (1 << 31)
+
+        while index < len(s) and s[index].isspace():
+            index += 1
+        if index == len(s):
+            return 0
+
+        if s[index] in ('-', '+'):
+            if s[index] == '-':
+                sign = -1
+            index += 1
+        if index == len(s):
+            return 0
+
+        final_integer = 0
+        while index < len(s) and s[index].isdigit():
+            digit = ord(s[index]) - ord('0')
+            final_integer = 10 * final_integer + sign * digit
+            if final_integer > MAX_INT:
+                return MAX_INT
+            if final_integer < MIN_INT:
+                return MIN_INT
+            index += 1
+
+        return final_integer
+```
+
+### 3回目
+```python
+class Solution:
+    def myAtoi(self, s: str) -> int:
+        index = 0
+        sign = 1
+        MAX_INT = (1 << 31) - 1
+        MIN_INT = - (1 << 31)
+
+        while index < len(s) and s[index].isspace():
+            index += 1
+        if index == len(s):
+            return 0
+
+        if s[index] in ('+', '-'):
+            if s[index] == '-':
+                sign = -1
+            index += 1
+        if index == len(s):
+            return 0
+
+        final_integer = 0
+        while index < len(s) and s[index].isdigit():
+            digit = ord(s[index]) - ord('0')
+            final_integer = 10 * final_integer + sign * digit
+            if final_integer > MAX_INT:
+                return MAX_INT
+            if final_integer < MIN_INT:
+                return MIN_INT
+            index += 1
+        return final_integer
+```
+
+### 4回目
+```python
+class Solution:
+    def myAtoi(self, s: str) -> int:
+        index = 0
+        sign = 1
+        MAX_INT = (1 << 31) - 1 
+        MIN_INT = - (1 << 31)
+
+        while index < len(s) and s[index].isspace():
+            index += 1
+
+        if index == len(s):
+            return 0
+
+        if s[index] in ('+', '-'):
+            if s[index] == '-':
+                sign = -1
+            index += 1
+
+        if index == len(s):
+            return 0
+
+        digits_integer = 0
+        while index < len(s) and s[index].isdigit():
+            digit = ord(s[index]) - ord('0')
+            digits_integer = 10 * digits_integer + sign * digit
+            if digits_integer > MAX_INT:
+                return MAX_INT
+            if digits_integer < MIN_INT:
+                return MIN_INT
+            index += 1
+
+        return digits_integer
+```