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Implement atoi which converts a string to an integer.

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.

Note:

• Only the space character ' ' is considered as whitespace character.
• Assume we are dealing with an environment which could only store integers within the 32-bit signed integer range: [−231,  231 − 1]. If the numerical value is out of the range of representable values, INT_MAX (231 − 1) or INT_MIN (−231) is returned.

将字符串转化为数字。通过样例来看，可以发现这道题情形较多，所以我分一些步骤来完成。

首先是用strip()去掉所有空格。

然后判断字符串长度是否不为0，第一个字符是否符合要求（属于‘+-0123456789')

如果以上条件满足了，开始遍历字符串：如果不属于’0123456789‘则中断并记录为止，如果完全遍历了，位置需要+1。

因为可能遇到“+”这种情况，没办法强制转换，所以用try方法解决速度比较快。

class Solution:    def myAtoi(self, str: str) -> int:        str=str.strip()        if len(str)==0 or str[0] not in '+-0123456789':return 0        loc=1        for i in range(1,len(str)):            if str[i] not in '0123456789':                loc=i                break            if i==len(str)-1:                loc=i+1        try:            res=int(str[:loc])            MAX = 2147483647            MIN = -2147483648            if res > MAX:                return MAX            if res < MIN:                 return MIN            return res        except:            return 0

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