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16. Best Time to Buy and Sell Stock

easyAsked at Instacart

Compute the max profit from one buy and one sell — Instacart uses this single-pass min-tracking warmup before pricing-window questions.

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

Problem

You are given an array prices where prices[i] is the price of a given stock on the i-th day. You want to maximize your profit by choosing a single day to buy and a different day in the future to sell. Return the max profit, or 0 if impossible.

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 all pairs

Try every (buy, sell) pair to find the max delta.

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 running min and best profit in one sweep.

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:

Instacart-specific tips

Instacart will reframe this as 'best window to promote a specific SKU' — keep your variable names neutral so it ports cleanly to the pricing context.

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Output

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