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12. Best Time to Buy and Sell Stock

easyAsked at Yelp

Find the maximum profit from a single buy and sell of a stock — Yelp uses this single-pass min-tracking pattern to test how candidates handle review-ranking score deltas over time.

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 from 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 (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 the lowest price seen so far and the best profit if we sold today.

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:

Yelp-specific tips

Yelp will reframe this as a review-ranking signal — be ready to discuss how a running minimum maps to detecting the largest delta in a business's rolling-window star score.

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Output

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