0008: String to Integer (atoi)

Problem Statement

Implement the myAtoi(string s) function, which converts a string to a 32-bit signed integer.

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

  1. Whitespace: Ignore any leading whitespace (" ").
  2. Signedness: Determine the sign by checking if the next character is '-' or '+', assuming positivity if neither present.
  3. Conversion: Read the integer by skipping leading zeros until a non-digit character is encountered or the end of the string is reached. If no digits were read, then the result is 0.
  4. Rounding: If the integer is out of the 32-bit signed integer range [-231, 231 - 1], then round the integer to remain in the range. Specifically, integers less than -231 should be rounded to -231, and integers greater than 231 - 1 should be rounded to 231 - 1.

Return the integer as the final result.

Example 1:

Input: s = "42"

Output: 42

Explanation:

The underlined characters are what is read in and 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)
           ^

Example 2:

Input: s = " -042"

Output: -42

Explanation:

Step 1: "   -042" (leading whitespace is read and ignored)
            ^
Step 2: "   -042" ('-' is read, so the result should be negative)
             ^
Step 3: "   -042" ("042" is read in, leading zeros ignored in the result)
               ^

Example 3:

Input: s = "1337c0d3"

Output: 1337

Explanation:

Step 1: "1337c0d3" (no characters read because there is no leading whitespace)
         ^
Step 2: "1337c0d3" (no characters read because there is neither a '-' nor '+')
         ^
Step 3: "1337c0d3" ("1337" is read in; reading stops because the next character is a non-digit)
             ^

Example 4:

Input: s = "0-1"

Output: 0

Explanation:

Step 1: "0-1" (no characters read because there is no leading whitespace)
         ^
Step 2: "0-1" (no characters read because there is neither a '-' nor '+')
         ^
Step 3: "0-1" ("0" is read in; reading stops because the next character is a non-digit)
          ^

Example 5:

Input: s = "words and 987"

Output: 0

Explanation:

Reading stops at the first non-digit character 'w'.

Constraints:

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

Code Solution

class Solution:
    def myAtoi(self, s: str) -> int:
	    
        ix = 0
        N = len(s)
        
        # remove whitespaces
        while ix < N and s[ix] == ' ':
            ix = ix + 1
        
        sign = +1
        
        # infer sign
        if ix < N and s[ix] in {'+', '-'}:
            sign = (+1 if s[ix] == '+' else -1)
            ix = ix + 1
        
        # read max non-digits
        out = 0
        while ix < N:
            if not s[ix].isdigit():
                break
            out = out * 10 + int(s[ix])
            ix = ix + 1
        
        # clamp and return number
        return min(max(sign * out, -(2 ** 31)), (2 ** 31) - 1)