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11. Best Time to Buy and Sell Stock

easyAsked at MercadoLibre

Find the largest single buy-then-sell profit from a price series.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

You are given an array prices where prices[i] is the price of an asset on day i. Find the maximum profit achievable from buying on one day and selling on a later day. Return 0 if no profit is possible.

Constraints

  • 1 <= prices.length <= 10^5
  • 0 <= prices[i] <= 10^4

Examples

Example 1

Input
prices = [7,1,5,3,6,4]
Output
5

Example 2

Input
prices = [7,6,4,3,1]
Output
0

Approaches

1. Brute force

Try every (buy, sell) pair.

Time
O(n^2)
Space
O(1)
let best = 0;
for (let i = 0; i < prices.length; i++)
  for (let j = i+1; j < prices.length; j++)
    best = Math.max(best, prices[j] - prices[i]);
return best;

Tradeoff:

2. Single pass min

Track the lowest price so far; each day, the answer is max(answer, today - lowest).

Time
O(n)
Space
O(1)
function maxProfit(prices) {
  let lo = Infinity, best = 0;
  for (const p of prices) {
    lo = Math.min(lo, p);
    best = Math.max(best, p - lo);
  }
  return best;
}

Tradeoff:

MercadoLibre-specific tips

Mercado Pago analysts ask this to gauge running-extremum thinking — it maps directly onto FX rate windows where buy/sell decisions feed remittance pricing across Argentina, Brazil, and Mexico.

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Output

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