leetcode: 8. String to Integer (atoi)

# Implement atoi to convert a string to an integer.
#
# Hint: Carefully consider all possible input cases.
# If you want a challenge, 
# please do not see below and ask yourself what are the possible input cases.
#
# Notes: 
# It is intended for this problem to be specified vaguely (ie, no given input specs).
# You are responsible to gather all the input requirements up front.
#
# Update (2015-02-10):
# The signature of the C++ function had been updated.
# If you still see your function signature accepts a const char * argument, 
# please click the reload button to reset your code definition.
#
# spoilers alert... click to show requirements for atoi.
#
# Requirements for atoi:
#
# 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.
# If the correct value is out of the range of representable values, 
# INT_MAX (2147483647) or INT_MIN (-2147483648) is returned.

class Solution():
    def myAtoi(self, str):
        INT_MAX =  2147483647
        INT_MIN = -2147483648
        i, sign, result = 0, 1, 0
        if not str:
            return result
        while i < len(str) and str[i].isspace():
            i += 1
        if str[i] == "+":
            i += 1
        elif str[i] == "-":
            sign = -1
            i += 1
        while i < len(str) and "0" <= str[i] <= "9":
            if result > (INT_MAX - int(str[i])) // 10:
                return INT_MAX if sign > 0 else INT_MIN
            result = result * 10 + int(str[i])
            i += 1
        return result * sign


if __name__ == '__main__':
    assert Solution().myAtoi("+-2") == 0
    assert Solution().myAtoi("") == 0
    assert Solution().myAtoi("-1") == -1
    assert Solution().myAtoi("2147483647") == 2147483647
    assert Solution().myAtoi("2147483648") == 2147483647
    assert Solution().myAtoi("-2147483648") == -2147483648
    assert Solution().myAtoi("-2147483649") == -2147483648