0125-valid-palindrome.js

/**
 * Array - Filter && Clone && Reverse
 * Time O(N) | Space O(N)
 * https://leetcode.com/problems/valid-palindrome/
 * @param {string} s
 * @return {boolean}
 */
var isPalindrome = function (s) {
    if (!s.length) return true;

    const alphaNumeric = filterAlphaNumeric(s); /* Time O(N) | Space O(N) */
    const reversed = reverse(alphaNumeric); /* Time O(N) | Space O(N) */

    return alphaNumeric === reversed;
};

const filterAlphaNumeric = (
    s,
    nonAlphaNumeric = new RegExp('[^a-z0-9]', 'gi'),
) =>
    s
        .toLowerCase() /* Time O(N) | Space O(N) */
        .replace(nonAlphaNumeric, ''); /* Time O(N) | Space O(N) */

const reverse = (s) =>
    s
        .split('') /* Time O(N) | Space O(N) */
        .reverse() /* Time O(N) | Space O(N) */
        .join(''); /* Time O(N) | Space O(N) */

/**
 * 2 Pointer | Midde Convergence
 * Time O(N) | Space O(1)
 * https://leetcode.com/problems/valid-palindrome/
 * @param {string} s
 * @return {boolean}
 */
var isPalindrome = function (s) {
    if (s.length <= 1) return true;

    let [left, right] = [0, s.length - 1];
    let leftChar, rightChar;
    while (left < right) {
        leftChar = s[left];
        rightChar = s[right];

        // skip char if non-alphanumeric
        if (!/[a-zA-Z0-9]/.test(leftChar)) {
            left++;
        } else if (!/[a-zA-Z0-9]/.test(rightChar)) {
            right--;
        } else {
            // compare letters
            if (leftChar.toLowerCase() != rightChar.toLowerCase()) {
                return false;
            }
            left++;
            right--;
        }
    }
    return true;
};

/**
 * 2 Pointer | Midde Convergence | No RegEx | No Copying
 * Time O(N) | Space O(1)
 * https://leetcode.com/problems/valid-palindrome/
 * @param {string} s
 * @return {boolean}
 */
var isPalindrome = function (s) {
    const isAlphaNumeric = (c) =>
        (c.toLowerCase() >= 'a' && c.toLowerCase() <= 'z') ||
        (c >= '0' && c <= '9');

    let left = 0;
    let right = s.length - 1;
    let skipLeft,
        skipRight,
        endsEqual = false;

    while (left < right) {
        skipLeft = !isAlphaNumeric(s.charAt(left));
        if (skipLeft) {
            left++;
            continue;
        }

        skipRight = !isAlphaNumeric(s.charAt(right));
        if (skipRight) {
            right--;
            continue;
        }

        endsEqual =
            s.charAt(left).toLowerCase() === s.charAt(right).toLowerCase();
        if (!endsEqual) return false;

        left++;
        right--;
    }
    return true;
};