Introduction

This problem is one of the most asked problems and it comes under the array manipulation topic. So if you have a good grasp of the array, this should be a piece of cake.
I am going to discuss 2 approaches here: The brute Force and the Optimized approach.

Prompt

You are given an array prices where prices[i] is the price of a given stock on the i<sup>th</sup> 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 = [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 before selling you need to buy a stock.

//Example 2:
Input: prices = [7,6,4,3,1]
Output: 0
Explanation: In this case, no transactions are done and the max profit = 0.

1) Brute Force Approach

const maxProfit = (prices) => {

    let buy = Infinity, profit = 0

    for (let i = 0; i < prices.length; i++) {
        let temp = -Infinity
        for (let j = i + 1; j < prices.length; j++) {
            temp = prices[j] - prices[i]
            profit = Math.max(profit, temp)
        }
    }
    return profit

};

const prices = [7,1,5,3,6,4]
const res = maxProfit(prices)
console.log(res) // 5

The time complexity for this approach will be O(n^2) and the space complexity is O(1).

Note: This brute force approach will throw a time limit exceeded error in LeetCode.

2) Optimized Approach

const maxProfit = (prices) => {

    let buy = Infinity, profit = 0

    for (let i = 0; i < prices.length; i++) {
        let current = prices[i]       // current selling price
        buy = Math.min(buy, current)  // to calculate min buy value
        let temp = current - buy      // getting current profit
        profit = Math.max(profit, temp) // to calculate max profit
    }

    return profit

};

const prices = [7,1,5,3,6,4]
const res = maxProfit(prices)
console.log(res) // 5

Ending Note

This is a classic array manipulation problem and is often asked during interviews.