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17. Best Time to Buy and Sell Stock

easyAsked at Spotify

Find the maximum profit from one buy and one sell of a stock.

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

Problem

Given an array prices where prices[i] is the price of a stock on day i, return the maximum profit you can achieve from one buy and one sell. Return 0 if no profitable trade exists.

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 pair (buy, sell) for max profit

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 min

Maintain the lowest price seen so far and update best profit each day. Single linear sweep.

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

Tradeoff:

Spotify-specific tips

Spotify treats this as a stand-in for streaming aggregates — the running-min pattern is identical to how their daily-active-user counters maintain a sliding low-water mark.

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Output

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