8. string conversion integer (ATOI)

8. String conversion integers (atoi)

Original title link ：https://leetcode.cn/problems/string-to-integer-atoi/

Please come to realize a myAtoi(string s) function , Enable it to convert a string to a 32 Bit signed integer （ similar C/C++ Medium atoi function ）.

function myAtoi(string s) The algorithm is as follows ：

Read in strings and discard useless leading spaces
Check the next character （ Suppose you haven't reached the end of the character yet ） Positive or negative , Read the character （ If there is ）. Determine whether the final result is negative or positive . If neither exists , Suppose the result is positive .
Read in the next character , Until you reach the next non numeric character or the end of the input . The rest of the string will be ignored .
Convert the numbers read in the previous steps into integers （ namely ,“123” -> 123, “0032” -> 32）. If you don't read in the numbers , Then the integer is 0 . Change the symbol if necessary （ From step 2 Start ）.
If the number of integers exceeds 32 Bit signed integer range [−231, 231 − 1] , You need to truncate this integer , Keep it in this range . say concretely , Less than −231 The integer of should be fixed to −231 , Greater than 231 − 1 The integer of should be fixed to 231 − 1 .
Returns an integer as the final result .
Be careful ：

The white space character in this question only includes the space character ’ ’ .
Except for the leading space or the rest of the string after the number , Do not ignore Any other character .

Example 1：

Input ：s = “42”
Output ：42
explain ： The bold string is the character that has been read in , The caret is the character currently read .
The first 1 Step ：“42”（ No characters are currently read in , Because there are no leading spaces ）
^
The first 2 Step ：“42”（ No characters are currently read in , Because it doesn't exist here ‘-’ perhaps ‘+’）
^
The first 3 Step ：“42”（ Read in “42”）
^
Parse to get an integer 42 .
because “42” In scope [-231, 231 - 1] Inside , The final result is 42 .
Example 2：

Input ：s = " -42"
Output ：-42
explain ：
The first 1 Step ：" -42"（ Read in leading space , But ignore ）
^
The first 2 Step ：" -42"（ Read in ‘-’ character , So the result should be negative ）
^
The first 3 Step ：" -42"（ Read in “42”）
^
Parse to get an integer -42 .
because “-42” In scope [-231, 231 - 1] Inside , The final result is -42 .
Example 3：

Input ：s = “4193 with words”
Output ：4193
explain ：
The first 1 Step ：“4193 with words”（ No characters are currently read in , Because there are no leading spaces ）
^
The first 2 Step ：“4193 with words”（ No characters are currently read in , Because it doesn't exist here ‘-’ perhaps ‘+’）
^
The first 3 Step ：“4193 with words”（ Read in “4193”; Because the next character is not a number , So read in stop ）
^
Parse to get an integer 4193 .
because “4193” In scope [-231, 231 - 1] Inside , The final result is 4193 .

Tips ：

0 <= s.length <= 200
s By the English letters （ Uppercase and lowercase ）、 Numbers （0-9）、’ ‘、’+‘、’-’ and ‘.’ form

Their thinking ：

Using regular expressions , Or use state automata algorithm .

Code implementation ：

Regular method ：

import re

class Solution:
    def myAtoi(self, s: str) -> int:
        #  Regular matching 
        matches = re.match('[ ]*([+-]?\d+)', s)
        if not matches:
            return 0
        #  Return matching content , The contents of regular hits are 1 No. capture group 
        res=int(matches.group(1))
        return min(max(res, -2**31), 2**31-1)

reference ：
https://leetcode.cn/problems/string-to-integer-atoi/solution/xiong-mao-shua-ti-python3-yi-qi-xue-xi-zheng-ze-bi/