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16. Best Time to Buy and Sell Stock

easyAsked at Booking

Pick the optimal buy/sell day pair — Booking probes this as a stand-in for finding the best snap-up window in a dynamic-pricing time series.

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

Problem

You are given an array prices where prices[i] is the price on day i. Find the maximum profit from one buy-and-sell pair; 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 pair (i, j) with j > i.

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 tracking

Track running min; profit = price - min.

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

Tradeoff:

Booking-specific tips

Booking maps this onto picking the best 'book-by' day in a dynamic-pricing series — explain how streaming a min-tracker fits a live availability + price feed.

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Output

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