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16. Best Time to Buy and Sell Stock

easyAsked at TripAdvisor

Find the maximum profit from buying and selling a stock once.

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

Problem

You are given an array prices where prices[i] is the price of a given stock on the i-th day. Find the maximum profit you can achieve by choosing a single day to buy one stock and choosing a different day in the future to sell it. 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 pairs

Try every (buy, sell) pair with i < j.

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. Track running minimum

Single pass: maintain min-so-far, candidate profit = price - min. Take max.

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

Tradeoff:

TripAdvisor-specific tips

TripAdvisor uses single-pass max-window problems to gauge if you can analyze price-history streams for hotel rate trends without nested loops.

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Output

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