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4. Best Time to Buy and Sell Stock

easyAsked at Postman

Find the maximum profit from one buy-sell transaction on a price timeline.

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

Problem

You are given an array prices where prices[i] is the stock price on day i. Choose a single day to buy and a later day to sell to maximize profit. 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 best 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]);

Tradeoff:

2. Single-pass min tracking

Walk once, tracking the lowest price seen so far and the best profit from selling today against that minimum.

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:

Postman-specific tips

Postman likes O(n) single-pass solutions because the API monitor pipeline scores request latency timelines the same way: one sweep, no replay.

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Output

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