lc008.把字符串转换成整数

class Solution:
    def strToInt(self, str: str) -> int:
        res, i, sign, length = 0, 0, 1, len(str)
        int_max, int_min, bndry = 2 ** 31 - 1, -2 ** 31, 2 ** 31 // 10
        if not str: return 0         # 空字符串，提前返回
        while str[i] == ' ':
            i += 1
            if i == length: return 0 # 字符串全为空格，提前返回
        if str[i] == '-': sign = -1
        if str[i] in '+-': i += 1
        for c in str[i:]:
            if not '0' <= c <= '9' : break
            if res > bndry or res == bndry and c > '7':
                return int_max if sign == 1 else int_min
            res = 10 * res + ord(c) - ord('0')
        return sign * res