LeetCode: 8. String to Integer (atoi)の解法

## 初めに
今回もLeetCodeをやっていきます。
今回の問題は以下

```
Implement the myAtoi(string s) function, which converts a string to a 32-bit signed integer.

The algorithm for myAtoi(string s) is as follows:

Whitespace: Ignore any leading whitespace (" ").
Signedness: Determine the sign by checking if the next character is '-' or '+', assuming positivity if neither present.
Conversion: Read the integer by skipping leading zeros until a non-digit character is encountered or the end of the string is reached. If no digits were read, then the result is 0.
Rounding: If the integer is out of the 32-bit signed integer range [-231, 231 - 1], then round the integer to remain in the range. Specifically, integers less than -231 should be rounded to -231, and integers greater than 231 - 1 should be rounded to 231 - 1.
Return the integer as the final result.

 

Example 1:

Input: s = "42"

Output: 42

Explanation:

The underlined characters are what is read in and the caret is the current reader position.
Step 1: "42" (no characters read because there is no leading whitespace)
         ^
Step 2: "42" (no characters read because there is neither a '-' nor '+')
         ^
Step 3: "42" ("42" is read in)
           ^
Example 2:

Input: s = " -042"

Output: -42

Explanation:

Step 1: "   -042" (leading whitespace is read and ignored)
            ^
Step 2: "   -042" ('-' is read, so the result should be negative)
             ^
Step 3: "   -042" ("042" is read in, leading zeros ignored in the result)
               ^
Example 3:

Input: s = "1337c0d3"

Output: 1337

Explanation:

Step 1: "1337c0d3" (no characters read because there is no leading whitespace)
         ^
Step 2: "1337c0d3" (no characters read because there is neither a '-' nor '+')
         ^
Step 3: "1337c0d3" ("1337" is read in; reading stops because the next character is a non-digit)
             ^
Example 4:

Input: s = "0-1"

Output: 0

Explanation:

Step 1: "0-1" (no characters read because there is no leading whitespace)
         ^
Step 2: "0-1" (no characters read because there is neither a '-' nor '+')
         ^
Step 3: "0-1" ("0" is read in; reading stops because the next character is a non-digit)
          ^
Example 5:

Input: s = "words and 987"

Output: 0

Explanation:

Reading stops at the first non-digit character 'w'.

 

Constraints:

0 <= s.length <= 200
s consists of English letters (lower-case and upper-case), digits (0-9), ' ', '+', '-', and '.'.
```

## 回答

```python
class Solution:
    def myAtoi(self, s: str) -> int:
        # 先頭の空白を除去
        s = s.lstrip()
        if not s:
            return 0
        
        sign = 1
        idx = 0
        
        # オプショナルな符号の処理
        if s[0] in ('+', '-'):
            # 符号の直後の文字が数字でなければ、変換失敗として 0 を返す
            if len(s) == 1 or not s[1].isdigit():
                return 0
            if s[0] == '-':
                sign = -1
            idx += 1
        
        result = ""
        while idx < len(s) and s[idx].isdigit():
            result += s[idx]
            idx += 1
        
        if result == "":
            return 0
        
        number = sign * int(result)
        # オーバーフローのチェック（必要なら）
        if number < -2**31:
            return -2**31
        if number > 2**31 - 1:
            return 2**31 - 1
        return number

```

## 最後に
普通に難しかったd