best-time-to-buy-and-sell-stock-ii.js

/**
 * Best Time to Buy and Sell Stock II
 * Say you have an array for which the ith element is the price of a given stock on day i.
 *
 * Design an algorithm to find the maximum profit. You may complete as many transactions as you like (i.e., buy one and sell one share of the stock multiple times).
 *
 * Note: You may not engage in multiple transactions at the same time (i.e., you must sell the stock before you buy again).
 *
 * Example 1:
 *
 * Input: [7,1,5,3,6,4]
 * Output: 7
 * Explanation: Buy on day 2 (price = 1) and sell on day 3 (price = 5), profit = 5-1 = 4.
 *              Then buy on day 4 (price = 3) and sell on day 5 (price = 6), profit = 6-3 = 3.
 *
 * Example 2:
 *
 * Input: [1,2,3,4,5]
 * Output: 4
 * Explanation: Buy on day 1 (price = 1) and sell on day 5 (price = 5), profit = 5-1 = 4.
 *              Note that you cannot buy on day 1, buy on day 2 and sell them later, as you are
 *              engaging multiple transactions at the same time. You must sell before buying again.
 *
 * Example 3:
 *
 * Input: [7,6,4,3,1]
 * Output: 0
 * Explanation: In this case, no transaction is done, i.e. max profit = 0.
 */

const assert = require('assert');

/**
 * @param {number[]} prices
 * @return {number}
 * @Complexity Analysis
 * Time: O(n)
 * Space: O(1)
 */
function maxProfit(prices) {
    let profit = 0;

    for (let i = 1, l = prices.length; i < l; i++) {
      if (prices[i] > prices[i - 1]) {
        profit += prices[i] - prices[i - 1];
      }
    }

    return profit;
}

/**
 * @param {number[]} prices
 * @return {number}
 * @Complexity Analysis
 * Time: O(n)
 * Space: O(1)
 */
function maxProfitPeakValley(prices) {
  let len = prices.length;
  let profit = 0;
  let i = 0;
  let valley;

  while (i < len - 1) {

    while (i < len - 1 && prices[i] >= prices[i + 1]) {
      i++;
    }

    valley = prices[i];

    while (i < len - 1 && prices[i] <= prices[i + 1]) {
      i++;
    }

    profit += prices[i] - valley;
  }


  return profit;
}


assert.deepStrictEqual(maxProfit([7,1,5,3,6,4]), 7);
assert.deepStrictEqual(maxProfit([1,2,3,4,5]), 4);
assert.deepStrictEqual(maxProfit([7,6,4,3,1]), 0);

assert.deepStrictEqual(maxProfitPeakValley([7,1,5,3,6,4]), 7);
assert.deepStrictEqual(maxProfitPeakValley([1,2,3,4,5]), 4);
assert.deepStrictEqual(maxProfitPeakValley([7,6,4,3,1]), 0);