LeetCode: [121] Best Time to Buy and Sell Stock

You are given an array prices where prices[i] is the price of a given stock on the ith day.

You want to maximize your profit by choosing a single day to buy one stock and choosing a different day in the future to sell that stock.

Return the maximum profit you can achieve from this transaction. If you cannot achieve any profit, return 0.

Example 1:

Input: prices = [7,1,5,3,6,4]

Output: 5

Explanation: Buy on day 2 (price = 1) and sell on day 5 (price = 6), profit = 6-1 = 5.

Note that buying on day 2 and selling on day 1 is not allowed because you must buy before you sell.

Example 2:

Input: prices = [7,6,4,3,1]

Output: 0

Explanation: In this case, no transactions are done and the max profit = 0.

Constraints:

• 1 <= prices.length <= 105
• 0 <= prices[i] <= 104

Solution

This problem is an ideal candidate for dynamic programming, which is keeping the best result until we see a better one.

In this case, we start from the end to the beginning, keeping track of the highest return and the highest stock price.

/**
 * @param {number[]} prices
 * @return {number}
 */
var maxProfit = function (prices) {
  // dp from end to start

  if (!prices || prices.length < 1) return 0

  let maxProfit = 0
  let high = 0

  for (let i = prices.length; i >= 0; i--) {
    let profit = high - prices[i]

    maxProfit = Math.max(maxProfit, profit)
    high = Math.max(prices[i], high)
  }

  return maxProfit
}

Source: https://leetcode.com/problems/best-time-to-buy-and-sell-stock/