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13. Best Time to Buy and Sell Stock

easyAsked at Activision

Find the maximum profit from one buy/sell pair — Activision uses this to gauge single-pass thinking before pivoting to leaderboard rating-delta problems.

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

Problem

Given an array of daily prices, return the maximum profit you could achieve from buying on one day and selling on a later day. If no profit is possible, return 0.

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

Check 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 tracking

Track running minimum price; max profit = best (price - min) seen so far in one sweep.

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

Tradeoff:

Activision-specific tips

Activision wants to see you collapse a two-pointer-looking problem into a single sweep — the same instinct maps to leaderboard rating-delta scans across a season window.

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Output

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