8.string-to-integer-atoi.py

#
# @lc app=leetcode id=8 lang=python3
#
# [8] String to Integer (atoi)
#
# https://leetcode.com/problems/string-to-integer-atoi/description/
#
# algorithms
# Medium (14.57%)
# Total Accepted:    346.1K
# Total Submissions: 2.4M
# Testcase Example:  '"42"'
#
# 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.
#
#


class Solution:
    def myAtoi(self, str: str) -> int:
        lookupTable = ('0', '1', '2', '3', '4', '5',
                       '6', '7', '8', '9', '-', '+')
        num = 0
        negativeFlag = False
        flagSet = False
        for val in str.strip():
            if val in lookupTable:
                index = lookupTable.index(val)
                if index == 10 and not flagSet:
                    negativeFlag = True
                    flagSet = True
                elif index == 11 and not flagSet:
                    flagSet = True
                elif (index == 10 and flagSet) or (index == 11 and flagSet):
                    break
                else:
                    num = num * 10 + lookupTable.index(val)
                    flagSet = True
            else:
                break
        if negativeFlag:
            num = -num
        if num > 2 ** 31 - 1:
            return 2 ** 31 - 1
        elif num < -2 ** 31:
            return - 2 ** 31
        else:
            return num