139-8. 字符串转换整数 (atoi)

请你来实现一个 atoi 函数，使其能将字符串转换成整数。(前两个我写的,效率不是很高)
class Solution(object):
    def myAtoi1(self, s):
        """
        :type s: str
        :rtype: int
        """
        temp = s.strip()
        if not temp:
            return 0

        if temp[0].isalpha():
            return 0

        symbol_list = ["+", "-"]
        ret_str = ""
        for i in range(len(temp)):
            if i == 0 and temp[i] in symbol_list:
                ret_str += temp[i]
                continue

            if not temp[i].isdigit():
                break
            ret_str += temp[i]

        ret_num = int(ret_str) if ret_str and ret_str not in symbol_list else 0
        if -2 ** 31 < ret_num < 2**31:
            return ret_num
        else:
            if ret_num > 0:
                return 2**31 - 1
            else:
                return -2 ** 31

    def myAtoi2(self, s):
        """
        :type s: str
        :rtype: int
        """
        temp = s.strip()
        if not temp:
            return 0

        if temp[0].isalpha():
            return 0

        symbol_list = ["+", "-"]
        symbol = ""
        start = 0
        end = 0
        for i in range(len(temp)):
            if i == 0 and temp[i] in symbol_list:
                symbol += temp[i]
                start += 1
                end += 1
                continue

            if not temp[i].isdigit():
                break
            end += 1

        ret_num = int(symbol + temp[start: end]) if temp[start: end] else 0
        if -2 ** 31 < ret_num < 2**31:
            return ret_num
        else:
            if ret_num > 0:
                return 2**31 - 1
            else:
                return -2 ** 31

    def myAtoi(self, s):
        """这个代码写的漂亮"""
        return max(min(int(*re.findall('^[+-]?d+', s.lstrip())), 2**31 - 1), -2**31)


if __name__ == '__main__':
    import re
    s1 = Solution()
    s = "3.14159"
    print(s1.myAtoi(s))