0647-palindromic-substrings.js

/**
 * Brut Force - Check All Substrings
 * Time O(N^3) | Space O(1)
 * https://leetcode.com/problems/palindromic-substrings/
 * @param {string} s
 * @return {number}
 */
var countSubstrings = (s, count = 0) => {
    for (let left = 0; left < s.length; left++) {
        /* Time O(N) */
        for (let right = left; right < s.length; right++) {
            /* Time O(N) */
            count += Number(isPalindrome(s, left, right)); /* Time O(N) */
        }
    }

    return count;
};

const isPalindrome = (s, left, right) => {
    while (left < right) {
        /* Time O(N) */
        const isEqual = s[left] === s[right];
        if (!isEqual) return false;

        left++;
        right--;
    }

    return true;
};

/**
 * DP - Bottom Up
 * Matrix - Tabulation
 * 2 Pointer - Slide Window
 * Time O(N^2) | Space O(N^2)
 * https://leetcode.com/problems/palindromic-substrings/
 * @param {string} s
 * @return {number}
 */
var countSubstrings = (s, count = 0) => {
    const tabu = initTabu(s); /* Time O(N * N)  | Space O(N *  N) */

    count += singleLetters(s, tabu); /* Time O(N)      | Space O(N *  N) */
    count += doubleLetters(s, tabu); /* Time O(N)      | Space O(N *  N) */
    count += multiLetters(s, tabu); /* Time O(N *  N) | Space O(N *  N) */

    return count;
};

const initTabu = (s) =>
    new Array(s.length)
        .fill() /* Space O(N) */
        .map(() => new Array(s.length).fill(false)); /* Space O(N) */

const singleLetters = (s, tabu, count = 0) => {
    for (let index = 0; index < s.length; index++) {
        /* Time O(N) */
        tabu[index][index] = true; /* Space O(N * N) */

        count += Number(tabu[index][index]);
    }

    return count;
};

const doubleLetters = (s, tabu, count = 0) => {
    for (let curr = 0; curr < s.length - 1; curr++) {
        /* Time O(N) */
        const next = curr + 1;
        const isEqual = s[curr] === s[next];

        tabu[curr][next] = isEqual; /* Space O(N * N) */
        count += Number(tabu[curr][next]);
    }

    return count;
};

const multiLetters = (s, tabu, count = 0) => {
    for (let window = 3; window <= s.length; window++) {
        /* Time O(N) */
        count += slideWindow(s, tabu, window); /* Time O(N) | Space O(N * N) */
    }

    return count;
};

const slideWindow = (s, tabu, window, count = 0) => {
    let [left, right] = [0, window - 1];

    while (right < s.length) {
        /* Time O(N) */
        const isTrue = tabu[left + 1][right - 1];
        const isEqual = s[left] === s[right];

        tabu[left][right] = isTrue && isEqual; /* Space O(N * N) */
        count += Number(tabu[left][right]);

        left++;
        right++;
    }

    return count;
};

/**
 * 2 Pointer - Expand Around Center
 * Time O(N^2) | Space O(1)
 * https://leetcode.com/problems/palindromic-substrings/
 * @param {string} s
 * @return {number}
 */
var countSubstrings = (s, count = 0) => {
    for (let i = 0; i < s.length; i++) {
        /* Time O(N) */
        const [odd, even] = [i, i + 1];
        /* odd-length: single character center */
        count += isPalindromeFromCenter(s, i, odd); /* Time O(N) */
        /* even-length: consecutive characters center */
        count += isPalindromeFromCenter(s, i, even); /* Time O(N) */
    }

    return count;
};

const isPalindromeFromCenter = (s, left, right, count = 0) => {
    const isInBounds = () => 0 <= left && right < s.length;
    while (isInBounds()) {
        /* Time O(N) */
        const isEqual = s[left] === s[right];
        if (!isEqual) break;

        count++;

        left--;
        right++;
    }

    return count;
};