Power button [-] 8. Small daily practice string conversion integer (atoi) (python)

8. string conversion integer (atoi)

Topic links: https://leetcode-cn.com/problems/string-to-integer-atoi/
Difficulty: Medium

Title Description

Atoi you to implement a function, it can convert a string to an integer.

First, the function will begin with a space character discard useless if necessary, until the find to the first non-space character so far.

When we find the first non-space character is a positive or negative number, the combination of the symbols as much as possible with consecutive numbers up later, as the sign of integer; if the first non-space character is figures, which directly after the continuous numeric characters are combined to form an integer.

In addition to the string after a valid integer part may also exist extra characters, these characters can be ignored, they should not affect a function.

Note: if the character string in the first non-space character is not a valid integer character string is empty or contains only white space character string, then you will not need to be a function of conversion.

In any case, if the function can not effectively convert, 0 is returned.

Description:
We assume that the size of the environment can store 32-bit signed integer, then the value range of [-2 ^{31 is} 2 ^{31 is} - 1] If the value exceeds this range, return INT_MAX (2 ^{31 is} -. 1) or INT_MIN (-2 ^{31 is} ).

Examples

Example 1:
Input: "42"
Output: 42

Example 2:
Input: "-42"
Output: -42
explained: the first non-blank character '-', it is a negative sign.
We will all digital consecutive negative number and later combined as much as possible, and finally get -42.

Example 3:
Input: "4193 words with"
Output: 4193
Explanation: converting the digital OFF '3', because the next character is not numeric.

Example 4:
Input: "987 words and"
Output: 0
Explanation: the first non-space character is a 'w', but it is not a positive number or negative number.
Therefore, the conversion can not be performed effectively.

Example 5:
Input: "-91283472332"
Output: -2147483648
explanation: the number "-91283472332" Over the range of 32-bit signed integer.
So return INT_MIN (-231).

Code

class Solution:
    def myAtoi(self, str: str) -> int:
        # str = str.replace(' ','')
        flag = 1    # 1表示正数
        head_num = 1
        num = 0
        for char in str:
            if char == ' ':
                if head_num == 1:
                    continue
                break
            elif char <= '9' and char >= '0' or char == '-' or char == '+':
                
                if char == '-' or char == '+':
                    if head_num == 0:
                        break
                    flag = -1 if char == '-' else 1
                    head_num = 0 
                else:
                    num = num*10 + int(char)
                    head_num = 0 
            else:
                break        
            
            
        if flag*num > 2**31-1:
            return flag*2**31-1
        elif flag*num < -2**31:
            return flag*2**31
        return flag*num