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7. Best Time to Buy and Sell Stock

easyAsked at Squarespace

Compute the maximum single-trade profit in a price series; Squarespace uses it to probe whether you reach for a one-pass min-tracker before reaching for nested loops.

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 achievable by buying 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

Check every pair of buy/sell days.

Time
O(n^2)
Space
O(1)
let best=0;
for (let i=0;i<n;i++)
  for (let j=i+1;j<n;j++)
    best=Math.max(best, prices[j]-prices[i]);
return best;

Tradeoff:

2. Single pass min tracker

Track minimum price so far and update the best profit on each step. One linear scan handles it.

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:

Squarespace-specific tips

Squarespace evaluators like to see you call out that the same min-tracker shape powers their daily-visitor analytics rollup in the site dashboard.

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Output

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