Leetcode – 8. String to Integer (atoi)

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

Problem

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.

Solution

Idea

My idea is straightforward. We just read character in the string one by one. If we have not encounter a number, there’re some special cases we need to handle:
– leading space/zero.
– positive/negative sign. also this sign will start a number.

Next we will check the character is number or not. If it’s a number, add it to the result string. If not, break the loop.

Before return the result, we need to handle the rounding then it’s good.

Code

/**
 * @param {string} s
 * @return {number}
 */
var myAtoi = function(s) {
    let numberStart = false;
    let result = '0';
    let positive = 1;
    for (let i = 0; i < s.length; i++) {
        const char = s[i];
        // if we have not encounter a number, we can skip leading space
        // also can check "-", "+" for negative/positive number
        if (!numberStart) {
            if (char === ' ' || char === '0') continue;
            if (char === '-') {
                numberStart = true;
                positive = -1;
                continue;
            }
            if (char === '+') {
                numberStart = true;
                continue;
            }
        }

        if (isCharNumber(char)) {
            numberStart = true;
            result = result.toString() + char;
        } else { // not a number --> break
            break;
        }
    }

    result = parseInt(result) * positive;
    if (result < -2147483648) return -2147483648;
    if (result > 2147483647) return 2147483647;
    return result;
};

function isCharNumber(c) {
  return c >= '0' && c <= '9';
}

Result