题目描述：
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.
代码：
1 class Solution: 2 def myAtoi(self, str: str) -> int: 3 str = str.strip() 4 strNum = 0 5 if len(str) == 0: 6 return strNum 7 8 positive = True 9 if str[0] == '+' or str[0] == '-': 10 if str[0] == '-': 11 positive = False 12 str = str[1:] 13 14 count = 0 15 for char in str: 16 if char >= '0' and char <= '9': 17 # strNum = strNum * 10 + ord(char) - ord('0') 18 count += 1 19 if char < '0' or char > '9': 20 break 21 22 # 区间是右开的，所以 :count只能取到count-1 23 # 如果字符串不能正常进行转换，要返回0，所以要首先对count的值进行判断，以确定转换是否有效， 24 # 否则无法排除掉转换无效的情况 25 if count != 0: 26 strNum = int(str[:count]) 27 28 if strNum > 2147483647: 29 if positive == False: 30 return -2147483648 31 else: 32 return 2147483647 33 if not positive: 34 strNum = 0 - strNum 35 return strNum 36 37 if __name__ == '__main__': 38 sol = Solution() 39 y = sol.myAtoi(" with words 4193") 40 print(y)
参考资料：
[1] Python中strip()函数的用法：https://www.cnblogs.com/yyxayz/p/4034299.html
[2] python中chr()函数和ord()函数的用法：https://www.cnblogs.com/chaojiyingxiong/p/9169458.html