JavaScript Challenge - Maximize Stock Trading Profit

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At first i thought it was as simple as to detect the lower and higher stock price in a “stockPrices” array and return the pair. Silly me, i then realized that the function should sell AFTER buying, so it should ignore high prices at the beginning and low prices at the end of the array haha.

It was a pretty entertaining project.

function maxProfitDays(stockPrices) { const detectBestDayToSell = (i, priceList) => { let higherPrice = priceList[i]; let day = i; for (let j = i++; j < priceList.length; j++) { if (priceList[j] > higherPrice) { higherPrice = priceList[j]; day = j } } return day } let bestDaysToBuyNSell = []; let bestProfit = 0; for (let buyDay = 0; buyDay < stockPrices.length; buyDay++) { const bestDayToSell = detectBestDayToSell(buyDay, stockPrices); const profit = stockPrices[bestDayToSell] - stockPrices[buyDay]; if (profit > bestProfit) { bestProfit = profit; bestDaysToBuyNSell = [buyDay, bestDayToSell]; } } return bestDaysToBuyNSell } // Testing const list = [120, 150, 17, 60, 25, 75, 31, 11]; console.log(maxProfitDays(list));

Here is my brute force solution by running 2 loops one on the day to buy the the second on the day to sell, by keeping day to sell after day to buy. it’s O(n^2).

function maxProfitDays(stockPrices) { let profit = 0; let buyDay; //the day we buy let sellDay; //the day we sell // loop day to buy i from day 0 to last day in the array for (let i=0; i<stockPrices.length; i++) { // loop day to sell from day i+1 to last day in the array for (let j=i+1; j<stockPrices.length; j++) { //test whether the profit on the 2 days is higher than the maximum previous profit if (stockPrices[j]-stockPrices[i] > profit) { profit = stockPrices[j]-stockPrices[i]; buyDay = i; sellDay = j; } } } return [buyDay,sellDay]; }; console.log(maxProfitDays([17, 11, 60, 25, 150, 75, 31, 120]))

A more optimal code with only one loop and 2 pointers: one starting from the beginning and one from the end. it’s an O(N). Please let me know what you think. I tested it on random lists of 500 elements, it gives the same result as the standard code, but i didn’t see any difference in computation time.

function maxProfitDays(stockPrices) { let buyDay; //the day we buy let sellDay; //the day we sell let min = stockPrices[0]; let max = stockPrices[stockPrices.length-1]; let i=0; let j = stockPrices.length-1; while (i<=j) { if (stockPrices[i] <= min) { min = stockPrices[i]; buyDay = i; i++; } else if (stockPrices[i] <= max) {i++}; if (stockPrices[j] >=max) { max = stockPrices[j]; sellDay = j; j--; } else if (stockPrices[j] >=min) {j--} if (stockPrices[i]> max && stockPrices[j] < min) { if (stockPrices[i] -min>= max-stockPrices[j]) {j--} else {i++}; } } return [buyDay,sellDay]; }; console.log(maxProfitDays([17, 11, 60, 25, 150, 75, 31, 120]))
1 Like

I really enjoyed this challenge.

Tested it too with some lists and it works fine. I’ll leave the explicit version just because i don’t like else statments that much, but regardless your code is so nice.

function maxProfitDays(stockPrices) { let buyDay; let sellDay; let i = 0; let j = stockPrices.length - 1; let min = stockPrices[i]; let max = stockPrices[j]; while (i <= j) { if (stockPrices[i] <= min) { min = stockPrices[i]; buyDay = i; i++ } if (stockPrices[i] > min && stockPrices[i] <= max) { i++ } if (stockPrices[j] >= max) { max = stockPrices[j]; sellDay = j; j-- } if (stockPrices[j] >= min) { j-- } if (stockPrices[i] > max && stockPrices[j] < min) { if (stockPrices[i] - min >= max - stockPrices[j]) { j-- } if (stockPrices[i] - min < max - stockPrices[j]) { i++ } } } return [buyDay,sellDay] }; console.log(maxProfitDays([17, 11, 60, 25, 150, 75, 31, 120]));

function maxProfitDays(n) {
let max = n[0];
let maxIndex = 0;
let min = n[0];
let minIndex = 0;
for (let j = 0; j < n.length; j++) {
if (n[j] > max) {
max = n[j];
maxIndex = j;
for (let i = 0; i<maxIndex; i++) {
if (n[i] < min) {
min = n[i];
minIndex = i;
return [minIndex, maxIndex];
////// I only get 3/5 test passed and i really dont know why, any help as to whats wrong ?? thanks in advance!!

I think the issue is that the index of the maximum could be less than the index of the minimum, so that’s potentially an answer that’s not viable.
The trickier thing is to find the maximum difference between a value at an index, and another value at a greater index (representing the sale say being after the buy day).

Hi Guys. For some reason I have tried several times my code with different arrray arrangements to check everything was fine but the tester keeps telling me there is an error. Dunno, by now, this is my answer. If someone may know where i might have an error, please comment.

function maxProfitDays(stockPrices) { let parCompraVenta = [0, 0]; let mapped =, i) { return { index: i, value: el }; }); mapped.sort(function(a, b) { if (a.value > b.value) { return 1; } if (a.value < b.value) { return -1; } return 0; }); let j = 0; let maxDiff = 0; mapped.forEach(pair =>{ while(j < stockPrices.length - 1){ if(mapped[j+1].index > pair.index){ if(mapped[j+1].value - pair.value > maxDiff){ maxDiff = mapped[j+1].value - pair.value; parCompraVenta = [pair.index, mapped[j+1].index]; } } j++; } }); return parCompraVenta; } console.log(maxProfitDays([17, 11, 60, 25, 150, 75, 31, 120])); // Leave this so we can test your code: module.exports = maxProfitDays;

Something similar happened to me. How did you check how many test you passed?

Should not this code work? it only passes one test.

const maxProfitDays = stockPrices => { const maximum = Math.max(...stockPrices); const minimum = Math.min(...stockPrices); return [stockPrices.indexOf(minimum), stockPrices.indexOf(maximum)]; } // Leave this so we can test your code: module.exports = maxProfitDays;

It is possible that the minimum has a higher index than the maximum, but so you could get an invalid result in that situation; because you can’t sell the stock before you buy it.

Hi All …
Is it possible that Codecademy challenge part is not handling objects properly ? I have this code and I know it works (at least i think it does :slight_smile: ) … But when i test it - it throws an error, or gives me -3/5 cases passed (like really 'minus 3 out of 5 cases passed) or -2/5 cases passed etc…

I think I’m getting paranoid :slight_smile:

let maxProfit = {
  profit: -Infinity,

function maxProfitDays(stockPrices) {
  for (let dayBuy = 0; dayBuy < stockPrices.length; dayBuy++) {
    for (let daySell = dayBuy + 1; daySell < stockPrices.length;daySell++) {
      let priceDifference = stockPrices[daySell] - stockPrices[dayBuy];
      if (priceDifference >= maxProfit.profit) {     
       Object.assign(maxProfit, {profit: priceDifference, indexBuy: dayBuy, indexSell: daySell});     
return [maxProfit.indexBuy, maxProfit.indexSell];

maxProfitDays([14, 17, 60, 25, 150, 75, 31, 120]);

// Leave this so we can test your code:
module.exports = maxProfitDays;