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6. Best Time to Buy and Sell Stock

easyAsked at Freshworks

Find the max single-pass profit — Freshworks dresses this up as 'lowest SLA breach risk window' across a day of ticket resolution times.

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

Problem

Given an array prices where prices[i] is the price on day i, return the maximum profit from buying one day and selling on a later day. 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

Try every (buy, sell) pair and track the max difference.

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 with running min

Track the lowest price so far; at each day, candidate profit is today minus running min. Keep the best.

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

Tradeoff:

Freshworks-specific tips

Freshworks interviewers like you to draw the prices as a time-series before coding — it mirrors how their SLA timer dashboards graph response curves.

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Output

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