LeetCode 121. Best Time to Buy and Sell Stock (javascript)

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 <= 10⁵
0 <= prices[i] <= 10⁴

Idea:

Tracking two variables – minStock(min price of the stock) and maxGain(max profit)

Solution:

/**
 * @param {number[]} prices
 * @return {number}
 */
var maxProfit = function(prices) {
    let maxGain = 0;
    let minStock = prices[0];
    
    for (let i = 0; i < prices.length; i++) {
        minStock = Math.min(minStock, prices[i]);
        maxGain = Math.max(maxGain, prices[i] - minStock);
    }
    return maxGain;
};