題目:
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.


解題思路

去除空白：跳過前面的空格。

處理符號：看下一個字元是否是 + 或 -，預設正號。

讀取數字：從第一個數字開始，持續轉換為整數，直到遇到非數字或字串結束。

num = num * 10 + (digit)

檢查溢位：

如果結果大於 Integer.MAX_VALUE (2^31 - 1) → 回傳 Integer.MAX_VALUE。

如果結果小於 Integer.MIN_VALUE (-2^31) → 回傳 Integer.MIN_VALUE。

輸出結果：套上符號後回傳。