121. Best Time to Buy and Sell Stock

Problem

Tags: Array, Dynamic Programming

You are given an array prices where prices[i] is the price of a given stock on the i^th 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 <= 10^5
• 0 <= prices[i] <= 10^4

Code

JS

// 121. Best Time to Buy and Sell Stock (6/17/53718)
// Runtime: 156 ms (4.64%) Memory: 52.42 MB (2.83%) 

/**
 * @param {number[]} prices
 * @return {number}
 */
function maxProfit(prices) {
    let highests = [],
        highest = -1;
    for (let i = prices.length - 1; i >= 0; i--) {
        if (prices[i] > highest) highest = prices[i];
        highests[i] = highest;
    }

    let max = 0;
    for (let i = 0; i < prices.length; i++) {
        let profit = highests[i] - prices[i];
        if (profit > max) max = profit;
    }

    return max;
}