121. Best Time to Buy and Sell Stock

Description

Say you have an array for which the i^th element is the price of a given stock on day i.

If you were only permitted to complete at most one transaction (ie, buy one and sell one share of the stock), design an algorithm to find the maximum profit.

Example 1:

Input: [7, 1, 5, 3, 6, 4]
Output: 5

max. difference = 6-1 = 5 (not 7-1 = 6, as selling price needs to be larger than buying price)

Example 2:

Input: [7, 6, 4, 3, 1]
Output: 0

In this case, no transaction is done, i.e. max profit = 0.

Solution

/**
 * @param {number[]} prices
 * @return {number}
 */
var maxProfit = function(prices) {
    "use strict";
    let profit = 0
    let min = prices[0], max = prices[0];
    for (let i = 1; i < prices.length; i++) {
        if (prices[i] < min) {
            // Current profit is greater than the previous one
            if (max-min > profit) {
                profit = max-min;
            }
            min = prices[i];
            // Find the max number greater than current index
            max = prices[i];
        }
        if (max < prices[i]) {
            max = prices[i];
        }
    }
    // Final profit is greater than the previous one
    if (max-min > profit) {
        profit = max-min;
    }
    return profit;
};