Leetcode String to Integer (atoi)

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本文介绍了一个将字符串转换为整数的功能函数实现，该函数首先移除字符串开头的所有空白字符，然后读取一个可选的正负号及后续的数字字符，将它们解析成数值。文章通过多个实例演示了不同输入情况下的处理方式。

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题目：String to Integer (atoi)

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.

Example 1:

Input: "42"
Output: 42

Example 2:

Input: "   -42"
Output: -42
Explanation: The first non-whitespace character is '-', which is the minus sign.
             Then take as many numerical digits as possible, which gets 42.

Example 3:

Input: "4193 with words"
Output: 4193
Explanation: Conversion stops at digit '3' as the next character is not a numerical digit.

Example 4:

Input: "words and 987"
Output: 0
Explanation: The first non-whitespace character is 'w', which is not a numerical 
             digit or a +/- sign. Therefore no valid conversion could be performed.

Example 5:

Input: "-91283472332"
Output: -2147483648
Explanation: The number "-91283472332" is out of the range of a 32-bit signed integer.
             Thefore INT_MIN (−231) is returned.

解答：

参考String to Integer (atoi)

class Solution:
    def myAtoi(self, str):
        """
        :type str: str
        :rtype: int
        """
        
        str = str.strip()
        if len(str) == 0:
            return 0
        index = 0
        sign = False
        if str[0] == '+' or str[0] == '-':
            index += 1
            sign = True
        for i in range(index, len(str)):
            if str[i] < '0' or str[i] > '9':
                break
            else:
                index += 1
        if index == 0:
            return 0
        if index == 1 and sign:
            return 0
        if int(str[:index]) > 2**31 - 1:
            return 2**31 - 1
        if int(str[:index]) < - 2**31:
            return -2 ** 31
        return int(str[:index])