StringToInteger

Problem

https://leetcode.com/problems/string-to-integer-atoi/

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 [-2:sup:`31`, 2:sup:`31`- 1], then round the integer to remain in the range. Specifically, integers less than -2:sup:`31` should be rounded to -2:sup:`31`, and integers greater than 2:sup:`31`- 1 should be rounded to 2:sup:`31`- 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 '.'.

Solution

Track the value of the integer as in separate sign and value variables.

1. Use s.strip() to remove any whitespace.

2. Check if the string is empty. If so, return 0.

3. Check the first character of the string. If it is ‘+’ or ‘-’, the number portion of the string starts at 1. Set the sign based on the character or default to a positive sign.

4. Get all the numeric digits in the string.

5. From the end, increase value by \(\\textrm{digit}_i * 10^i\). If value exceeds the int32 limits of \([-2^{32}, 2^{32} - 1]\), clamp value.

Pattern

String

Code

def myAtoi(s: str) -> int:
    """Convert string to integer.
    """
    sign = 1
    value = 0

    s = s.strip()

    if len(s) == 0:
        return 0

    sign_char = s[0]
    num_start = 0
    if sign_char in {'-', '+'}:
        num_start = 1
    if sign_char == '-':
        sign = -1

    num_string = ''
    for char in s[num_start:]:
        if not char.isnumeric():
            break

        num_string += char

    for i, char in enumerate(reversed(num_string)):
        value += int(char) * 10**i
        if sign == 1 and value >= 2**31 - 1:
            return 2**31 - 1
        elif sign == -1 and value >= 2**31:
            return -2**31

    return sign * value

Test

>>> from StringToInteger import myAtoi
>>> myAtoi('42')
42
>>> myAtoi('   -42')
-42
>>> myAtoi('4193 with words')
4193

StringToInteger.myAtoi(s: str) → int

Convert string to integer.