8. String to Integer (atoi).py

"""
https://leetcode.com/problems/string-to-integer-atoi/description/

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'.
"""


class Solution:
    def myAtoi(self, s: str) -> int:
        result = 0
        s = s.lstrip()
        if not s:
            return result
        is_negative = s[0] == "-"
        if not s[0].isdigit():
            s = s[1:]
        for ch in s:
            if ch.isdigit():
                result *= 10
                result += int(ch)
            else:
                break
        return max(-result, (-2) ** 31) if is_negative else min(result, 2 ** 31 - 1)


solution = Solution()

assert solution.myAtoi("42") == 42
assert solution.myAtoi("-042") == -42
assert solution.myAtoi("1337c0d3") == 1337
assert solution.myAtoi("0-1") == 0
assert solution.myAtoi("words and 987") == 0