8. String to Integer (atoi).py

# 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: [−231,  231 − 1]. If the numerical value is out of the range of representable values, INT_MAX (231 − 1) or INT_MIN (−231) 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 (−231) is returned.

INT_MAX = 2147483647
INT_MIN = -2147483648

class Solution:
    def strToInt(self, str: str) -> int:
        ret = 0
        stopLen = 0
        while len(str) > 0:
            if not str[0].isdecimal():
                stopLen = len(str)
                return int(ret / 10**stopLen)
            else:
                ret += int(str[0]) * 10**(len(str)-1)
                str = str[1:]

        return ret
    
    def myAtoi(self, s: str) -> int:
#         ret = 0
#         str = str.strip()
#         if str == '':
#             return 0
        
#         if str[0].isdigit():
#             return self.strToInt(str)
#         elif str[0].isalpha():
#             return 0
#         elif (str[0] == '-') or (str[0] == '+'):
#             if str[0] == '+':
#                 if self.strToInt(str[1:]) >= 2147483647:
#                     return 2147483647
#                 else:
#                     return self.strToInt(str[1:])
#             elif str[0] == '-':               
#                 if -self.strToInt(str[1:]) <= INT_MIN:
#                     return INT_MIN
#                 else:
#                     return -self.strToInt(str[1:])
#         else:
#             return 0
        return max(min(int(*re.findall('^[\+\-]?\d+', s.lstrip())), 2**31 - 1), -2**31)