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):

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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.

实现atoi函数将String转换成Integer。

提示：仔细考虑所有可能的输入情况。如果你想挑战一下自己，就不要看下面的提示，问一下自己可能的输入有哪些。

注意：这个问题比较模糊（ie，没有指定输入规格）。你要预先考虑到所有的输入情况。

atoi需求：

函数首先将开始处的所有空格删除，直到出现第一个非空格字符。然后确定接下来的n个数字字符的符号，并将其转换成数字。

字符串可能包含额外的非数字的字符，应该舍弃。

如果第一个非空格字符序列不是合法的数字，或者这样的序列根本不存在（字符串本身为空或者只包含空格）就不进行转换。

如果不进行转换就返回0。如果转换后的值超出了可表示的范围，返回

INT_MAX (2147483647)

或者

INT_MIN (-2147483648)