Skip to main content

16. Best Time to Buy and Sell Stock

easyAsked at Slack

Maximize a single buy/sell profit from a daily price array.

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 ith day. Choose one day to buy and a future day to sell. Return the maximum profit; if none 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 tracker

Track running min price; for each day, compute profit if sold today. Take the max.

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

Tradeoff:

Slack-specific tips

Slack's billing team likes profit-window problems — talk through how you'd extend this to multi-month subscription pricing windows.

Solve it now

Free. No sign-up. Python and JavaScript run instantly in your browser.

Output

Press Run or Cmd+Enter to execute

Practice these live with InterviewChamp.AI

Drill Best Time to Buy and Sell Stock and other Slack interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →