String to Integer (atoi)

Problem

Implement atoi which converts a string to a 32-bit signed integer.

Rules:

Ignore leading whitespace.
Optional + or - determines the sign.
Parse digits until the first non-digit character (or end of string).
Clamp the result to $[-2^{31}, 2^{31} - 1]$ on overflow.

Case 1

Input: s = "42"
Output: 42

Case 2

Input: s = "   -42"
Output: -42

Case 3

Input: s = "4193 with words"
Output: 4193

Constraints

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

Solution

Deterministic Parser

function myAtoi(s: string): number {
  const INT_MIN = -(2 ** 31);
  const INT_MAX = 2 ** 31 - 1;

  let i = 0;
  const n = s.length;

  while (i < n && s[i] === " ") i++; // skip leading spaces
  if (i === n) return 0;

  let sign = 1;
  if (s[i] === "+" || s[i] === "-") {
    sign = s[i] === "-" ? -1 : 1;
    i++;
  }

  let result = 0;
  while (i < n && s[i] >= "0" && s[i] <= "9") {
    const digit = s.charCodeAt(i) - "0".charCodeAt(0);
    if (
      result > Math.floor(INT_MAX / 10) ||
      (result === Math.floor(INT_MAX / 10) && digit > INT_MAX % 10)
    ) {
      return sign === 1 ? INT_MAX : INT_MIN;
    }
    result = result * 10 + digit;
    i++;
  }

  return result * sign;
}

This parser walks the string once: trimming whitespace, capturing an optional sign, and multiplying the running value by 10 as long as digits appear. The overflow guard compares against INT_MAX before adding a new digit, returning the saturated limit if necessary. Time complexity is $O(n)$ and space overhead is $O(1)$.