[Leetcode]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.

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.

把字符串转换成数字~要注意细节的处理，如负数处理，还有整数越界的情况~先把多余的空格去掉，读取正负符号，然后读取数字，注意数字的后面可能包含一些异常字符，此时可以忽略这些异常字符，保留前面的结果就行~还有，如果得到的整数如果越界了，返回INT_MAX或INT_MIN~

class Solution:    # @return an integer    def atoi(self, str):        if str is None or len(str) == 0: return 0        i, res, sign = 0, 0, 1        while i < len(str) and str[i] == ' ':            i += 1        if str[i] == '+':            i += 1        elif str[i] == '-':            i += 1            sign = -1        while i < len(str) and str[i] >= '0' and str[i] <= '9':            res = res * 10 + int(str[i])            i += 1        return max(min(res * sign, 2147483647), -2147483648)