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7. Best Time to Buy and Sell Stock

easyAsked at Klarna

Find the maximum profit from a single buy-sell pair over a price series.

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

Problem

Given an array prices where prices[i] is the price on day i, find the maximum profit 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

Check every (buy, sell) pair.

Time
O(n^2)
Space
O(1)
function maxProfit(prices) {
  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 with running min

Track the minimum price seen so far and the best profit if we sold today. One pass, constant memory.

Time
O(n)
Space
O(1)
function maxProfit(prices) {
  let minPrice = Infinity, best = 0;
  for (const p of prices) {
    if (p < minPrice) minPrice = p;
    else if (p - minPrice > best) best = p - minPrice;
  }
  return best;
}

Tradeoff:

Klarna-specific tips

Klarna risk teams love this pattern because it mirrors how their credit-scoring features compute running-min/running-max statistics on a customer's transaction history in a single sweep.

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Output

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