0091-decode-ways.js

/**
 * DP - Top Down
 * Hash Map - Memoization
 * Time O(N) | Space O(N)
 * https://leetcode.com/problems/decode-ways/
 * @param {string} s
 * @return {number}
 */
var numDecodings = (str, index = 0, memo = new Map()) => {
    const isBaseCase1 = !str.length || str[index] === '0';
    if (isBaseCase1) return 0;

    const isisBaseCase2 = index === str.length;
    if (isisBaseCase2) return 1;

    if (memo.has(index)) return memo.get(index);

    return dfs(str, index, memo);
};

const dfs = (str, index, memo) => {
    let count = numDecodings(str, index + 1, memo);

    if (isTwoDigit(str, index)) {
        count += numDecodings(str, index + 2, memo);
    }

    memo.set(index, count);

    return count;
};

var isTwoDigit = (str, index) => {
    const twoDigit = Number(str.slice(index, index + 2));

    return 10 <= twoDigit && twoDigit <= 26;
};

/**
 * DP - Bottom Up
 * Array - Tabulation
 * Time O(N) | Space O(N)
 * https://leetcode.com/problems/decode-ways/
 * @param {string} s
 * @return {number}
 */
var numDecodings = (s) => {
    const isBaseCase = !s.length || s[0] === '0';
    if (isBaseCase) return 0;

    const tabu = getTabu(s);

    decode(s, tabu);

    return tabu[s.length];
};

const getTabu = (s) => {
    const tabu = new Array(s.length + 1).fill(0);

    tabu[0] = 1;
    tabu[1] = s[1] === '0' ? 0 : 1;

    return tabu;
};

var decode = (s, tabu) => {
    for (let curr = 2; curr < tabu.length; curr++) {
        const [prev, prevPrev] = [curr - 1, curr - 2];
        const isEqual = s[prev] === '0';
        if (!isEqual) tabu[curr] += tabu[prev];

        if (isTwoDigit(s, curr)) tabu[curr] += tabu[prevPrev];
    }
};

var isTwoDigit = (s, index) => {
    const twoDigit = Number(s.slice(index - 2, index));

    return 10 <= twoDigit && twoDigit <= 26;
};

/**
 * 2 Pointer - previous + previousPrevious
 * Time O(N) | Space O(1)
 * https://leetcode.com/problems/decode-ways/
 * @param {string} s
 * @return {number}
 */
var numDecodings = (s) => {
    const isBaseCase = !s.length || s[0] === '0';
    if (isBaseCase) return 0;

    return decode(s);
};

var decode = (s) => {
    let [prev, prevPrev] = [1, 1];

    for (let curr = 1; curr < s.length; curr++) {
        const temp = prev;

        const isEqual = s[curr] === '0';
        if (isEqual) prev = 0;

        if (isTwoDigit(s, curr)) prev += prevPrev;

        prevPrev = temp;
    }

    return prev;
};

var isTwoDigit = (s, i) => {
    const [prevChar, curChar] = [s[i - 1], s[i]];
    const is10 = prevChar === '1';
    const is20 = prevChar === '2' && curChar <= '6';

    return is10 || is20;
};