Daily classic

Goshu Song — Western Borderers of tang Dynasty (Tang Dynasty)

Big Dipper high, Ge Shu night with a knife.

So far peep horse, did not dare lintao.

describe

Implement the myAtoi(string s) function, which converts a string to a 32-bit signed integer (similar to C/C++’s atoi function).

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

  • Read in and ignore any leading whitespace.
  • Check if the next character (if not already at the end of the string) is ‘-‘ or ‘+’. Read this character in if it is either. This determines if the final result is negative or positive respectively. Assume the result is positive if neither is present.
  • Read in next the characters until the next non-digit character or the end of the input is reached. The rest of the string is ignored.
  • Convert these digits into an integer (i.e. “123” -> 123, “0032” -> 32). If no digits were read, then the integer is 0. Change the sign as necessary (from step 2).
  • If the integer is out of the 32-bit signed integer range [-231, 231 – 1], then clamp the integer so that it remains in the range. Specifically, integers less than -231 should be clamped to -231, and integers greater than 231 – 1 should be clamped to 231 – 1.
  • Return the integer as the final result.

Note:

  • Only the space character ‘ ‘ is considered a whitespace character.
  • Do not ignore any characters other than the leading whitespace or the rest of the string after the digits.

Example 1:

Input: s = "42"
Output: 42
Explanation: The underlined characters are what is read in, the caret is the current reader position.
Step 1: "42" (no characters read because there is no leading whitespace)
         ^
Step 2: "42" (no characters read because there is neither a '-' nor '+')
         ^
Step 3: "42" ("42" is read in)
           ^
The parsed integer is 42.
Since 42 is in the range [-231, 231 - 1], the final result is 42.
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Example 2:

Input: s = "   -42"
Output: -42
Explanation:
Step 1: "   -42" (leading whitespace is read and ignored)
            ^
Step 2: "   -42" ('-' is read, so the result should be negative)
             ^
Step 3: "   -42" ("42" is read in)
               ^
The parsed integer is -42.
Since -42 is in the range [-231, 231 - 1], the final result is -42.
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Example 3:

Input: s = "4193 with words" Output: 4193 Explanation: Step 1: "4193 with words" (no characters read because there is no leading whitespace) ^ Step 2: "4193 with words" (no characters read because there is neither a '-' nor '+') ^ Step 3: "4193 with words" ("4193" is read in; reading stops because the next character is a non-digit) ^ The parsed integer is 4193. Since 4193 is in the range [-231,  231 - 1], the final result is 4193.Copy the code

Note:

  • 0 <= s.length <= 200
  • s consists of English letters (lower-case and upper-case), digits (0-9), ‘ ‘, ‘+’, ‘-‘, and ‘.’.

parsing

MyAtoi (string s) converts a string to a 32-bit signed integer. MyAtoi (string s) has the following algorithm:

  • Ignore any leading whitespace
  • Then check whether the next character (if not at the end of the string) is a “-” or a “+”. If so, read in this character to determine whether the final result is negative or positive. If neither exists, the result is assumed to be positive
  • The next character is read until the end of the next non-numeric character or input is reached, and the rest of the string is ignored.
  • Convert these numbers to integers (that is, “123” -> 123, “0032” -> 32). If no numbers are read, the integer is 0. Change symbols as needed (starting with Step 2).
  • If an integer exceeds the range of 32-bit signed integers [-231, 231-1], integers less than -231 should be limited to -231 and integers greater than 231-1 to 231-1.
  • Returns an integer as the final result.

In fact, there are a lot of boundary cases in this problem, which needs to be paid attention to, the rest only need to be written from front to back according to the above algorithm, I think the Acceptance of this problem is only 16%, maybe many people write codes according to their own ideas, so many boundary cases are reported wrong, because I am just like this.

answer

class Solution(object):
    def myAtoi(self, s):
        """
        :type s: str
        :rtype: int
        """
        if not s : return 0
        sign = 1
        s = s.lstrip()
        if not s: return 0
        if s[0] == "+" or s[0] == "-":
            sign = 1 if s[0] == "+" else  -1 
            s = s[1:]
        print(s)
        if not s : return 0
        result = 0
        for i in range(len(s)):
            digit = ord(s[i])-ord('0')
            if digit<0 or digit>9: break
            result = result * 10 + digit
    
        result *= sign
        if result>0:
            return result if result<=pow(2,31)-1 else pow(2,31)-1
        return result if result>-pow(2,31) else -pow(2,31)
            
        
        
        	      
		
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The results

Given in the linked list in the Python online submission. Given in the Python online submissions for String to Integer (AToi).Copy the code

Original link: leetcode.com/problems/st…

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