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16. Best Time to Buy and Sell Stock

easyAsked at CircleCI

Find the maximum profit from one stock transaction, testing the single-pass minimum-tracking pattern used in CI job cost optimization at CircleCI.

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 you can achieve from one buy-sell transaction. 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 all pairs of buy and sell days.

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

Tradeoff:

2. Single-pass min tracking

Track the minimum price seen so far and compute profit against the current price, updating max profit in one pass.

Time
O(n)
Space
O(1)
function maxProfit(prices) {
  let minPrice = Infinity;
  let maxProfit = 0;
  for (const price of prices) {
    if (price < minPrice) {
      minPrice = price;
    } else {
      maxProfit = Math.max(maxProfit, price - minPrice);
    }
  }
  return maxProfit;
}

Tradeoff:

CircleCI-specific tips

CircleCI uses this pattern to discuss build-cost windows — explain Kadane's algorithm connection and be ready to generalize to k transactions.

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