继续刷题

atoi函数的实现，需要注意的问题：

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.

1.从第一个不是空格的开始转换

2.如果空串，或者全是空格，或者没有符号+数字这种组合出现，返回0

3.注意正负和int越界的问题

4.连续符号出现返回0

5.转换过程中如果遇到其他非数字字符，停止转换，返回现有值