0070-climbing-stairs.js

/**
 * Brute Force - DFS
 * Time O(2^N) | Space O(N)
 * https://leetcode.com/problems/climbing-stairs/
 * @param {number} n
 * @return {number}
 */
var climbStairs = (n, index = 0) => {
    const isBaseCase1 = n < index;
    if (isBaseCase1) return 0;

    const isBaseCase2 = index === n;
    if (isBaseCase2) return 1;

    const [next, nextNext] = [index + 1, index + 2];
    const left = climbStairs(n, next); /* Time O(2^N) | Space O(N) */
    const right = climbStairs(n, nextNext); /* Time O(2^N) | Space O(N) */

    return left + right;
};

/**
 * DP - Top Down
 * Array - Memoization
 * Time O(N) | Space O(N)
 * https://leetcode.com/problems/climbing-stairs/
 * @param {number} n
 * @return {number}
 */
var climbStairs = (n, index = 0, memo = Array(n + 1).fill(0)) => {
    const isBaseCase1 = n < index;
    if (isBaseCase1) return 0;

    const isBaseCase2 = index === n;
    if (isBaseCase2) return 1;

    const hasSeen = memo[index] !== 0;
    if (hasSeen) return memo[index];

    const [next, nextNext] = [index + 1, index + 2];
    const left = climbStairs(n, next, memo); /* Time O(N) | Space O(N) */
    const right = climbStairs(n, nextNext, memo); /* Time O(N) | Space O(N) */

    memo[index] = left + right; /*           | Space O(N) */
    return memo[index];
};

/**
 * DP - Bottom Up
 * Array - Tabulation
 * Time O(N) | Space O(N)
 * https://leetcode.com/problems/climbing-stairs/
 * @param {number} n
 * @return {number}
 */
var climbStairs = (n) => {
    const isBaseCase = n === 1;
    if (isBaseCase) return 1;

    const tabu = initTabu(n); /* Space O(N) */

    search(n, tabu);

    return tabu[n];
};

var initTabu = (n) => {
    const tabu = new Array(n + 1).fill(0);

    tabu[1] = 1;
    tabu[2] = 2;

    return tabu;
};

var search = (n, tabu) => {
    for (let index = 3; index <= n; index++) {
        /* Time O(N) */
        const [prev, prevPrev] = [index - 1, index - 2];

        tabu[index] = tabu[prev] + tabu[prevPrev]; /* Space O(N) */
    }
};

/**
 * DP - Fibonacci Number
 * Time O(N) | Space O(1)
 * https://leetcode.com/problems/climbing-stairs/
 * @param {number} n
 * @return {number}
 */
var climbStairs = (n) => {
    const isBaseCase = n === 1;
    if (isBaseCase) return 1;

    let [next, nextNext] = [1, 2];

    for (let index = 3; index <= n; index++) {
        /* Time O(N) */
        const temp = next + nextNext;

        next = nextNext;
        nextNext = temp;
    }

    return nextNext;
};

/**
 * Matrix - Bitnets Method
 * Time O(log(N)) | Space O(1)
 * https://leetcode.com/problems/climbing-stairs/
 * @param {number} n
 * @return {number}
 */
var climbStairs = (n) => {
    const prev = [
        [1, 1],
        [1, 0],
    ];
    const next = power(n, prev); /* Time O(log(N)) */

    return next[0][0];
};

const power = (n, prev) => {
    let next = [
        [1, 0],
        [0, 1],
    ];

    const isEmpty = () => n === 0;
    while (!isEmpty()) {
        /* Time O(log(N)) */
        const isBit = (n & 1) === 1;
        if (isBit) next = multiply(next, prev); /* Time O(1) | Space O(1) */

        n >>= 1;
        prev = multiply(prev, prev); /* Time O(1) | Space O(1) */
    }

    return next;
};

const multiply = (prev, next) => {
    const [rows, cols] = [2, 2];
    const matrix = new Array(rows).fill().map(() => new Array(cols).fill(0));

    for (let row = 0; row < rows; row++) {
        for (let col = 0; col < cols; col++) {
            const left = prev[row][0] * next[0][col];
            const right = prev[row][1] * next[1][col];

            matrix[row][col] = left + right;
        }
    }

    return matrix;
};

/**
 * Math - Fibonacci Formula
 * Time O(log(N)) | Space O(1)
 * https://leetcode.com/problems/climbing-stairs/
 * @param {number} n
 * @return {number}
 */
var climbStairs = (n, sqrt5 = Math.sqrt(5)) => {
    const phi = (sqrt5 + 1) / 2;
    const psi = (sqrt5 - 1) / 2;

    const phiPow = Math.pow(phi, n + 1);
    const psiPow = Math.pow(psi, n + 1);

    return (phiPow - psiPow) / sqrt5;
};