0008.py

# Source: https://leetcode.com/problems/string-to-integer-atoi
# Title: String to Integer (atoi)
# Difficulty: Medium
# Author: Mu Yang <http://muyang.pro>

################################################################################################################################
# Implement atoi which converts a string to an integer.
#
# The function first discards as many whitespace characters as necessary until the first non-whitespace character is found. Then, starting from this character, takes an optional initial plus or minus sign followed by as many numerical digits as possible, and interprets them as a numerical value.
#
# The string can contain additional characters after those that form the integral number, which are ignored and have no effect on the behavior of this function.
#
# If the first sequence of non-whitespace characters in str is not a valid integral number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed.
#
# If no valid conversion could be performed, a zero value is returned.
#
# Note:
#
#   Only the space character ' ' is considered as whitespace character.
#   Assume we are dealing with an environment which could only store integers within the 32-bit signed integer range: [−2^31,  2^31 − 1]. If the numerical value is out of the range of representable values, INT_MAX (2^31 − 1) or INT_MIN (−2^31) is returned.
#
# Example 1:
#
#   Input: "42"
#   Output: 42
#
# Example 2:
#
#   Input: "   -42"
#   Output: -42
#   Explanation: The first non-whitespace character is '-', which is the minus sign.
#                Then take as many numerical digits as possible, which gets 42.
#
# Example 3:
#
#   Input: "4193 with words"
#   Output: 4193
#   Explanation: Conversion stops at digit '3' as the next character is not a numerical digit.
#
# Example 4:
#
#   Input: "words and 987"
#   Output: 0
#   Explanation: The first non-whitespace character is 'w', which is not a numerical
#                digit or a +/- sign. Therefore no valid conversion could be performed.
#
# Example 5:
#
#   Input: "-91283472332"
#   Output: -2147483648
#   Explanation: The number "-91283472332" is out of the range of a 32-bit signed integer.
#                Thefore INT_MIN (−2^31) is returned.
#
################################################################################################################################

import re

INT_MAX = 2**31-1
INT_MIN = -2**31

class Solution:
    def myAtoi(self, s: str) -> int:
        m = re.findall(r'^[+\-]?\d+', s.lstrip())

        if not m:
            return 0

        ans = int(m[0])

        if ans > INT_MAX:
            ans = INT_MAX
        elif ans < INT_MIN:
            ans = INT_MIN

        return ans