Skip to main content

17. Best Time to Buy and Sell Stock

easyAsked at Workday

Given daily stock prices, find the maximum profit from a single buy-and-sell. Workday uses this to test running-minimum tracking — the same pattern used for finding the best fiscal-year window in compensation grading.

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

Source citations

Public interview reports confirming this problem appears in Workday loops.

  • Glassdoor (2025)— Workday SDE1/2 phone screen — classic DP intro.
  • LeetCode Discuss (2026)— Workday compensation team interview.

Problem

You are given an array prices where prices[i] is the price of a given stock on the ith day. You want to maximize your profit by choosing a single day to buy one stock and choosing a different day in the future to sell that stock. Return the maximum profit you can achieve from this transaction. If you cannot achieve any profit, 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

Explanation: Buy on day 2 (price = 1), sell on day 5 (price = 6), profit = 6-1 = 5.

Example 2

Input
prices = [7,6,4,3,1]
Output
0

Explanation: No transaction yields profit.

Approaches

1. Brute force pairs

Try every buy-sell pair (i, j) with i < j.

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: Quadratic. Fine for n=100, dies at n=10^5.

2. Running minimum

Track the lowest price seen so far. For each day, profit if sold today = price - min. Keep the best.

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

Tradeoff: Single pass. The 'else if' guards against computing profit on the same day you set the new minimum (which is zero anyway, but cleaner).

Workday-specific tips

Workday wants you to explicitly say 'I'll track the running minimum and compute the gap'. Don't over-engineer with DP arrays for state[buy/sell] — those are LC 122/123. For one transaction, the one-pass minimum is canonical.

Common mistakes

  • Returning the price difference instead of the profit (could be negative).
  • Initializing minSoFar to 0 — fails when all prices > 0.
  • Returning -1 on no-profit instead of 0.

Follow-up questions

An interviewer at Workday may pivot to one of these next:

  • Best Time to Buy and Sell Stock II (LC 122) — unlimited transactions.
  • Best Time to Buy and Sell Stock III (LC 123) — at most 2 transactions.
  • Cooldown variant (LC 309).

Solve it now

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

Output

Press Run or Cmd+Enter to execute

FAQ

Why initialize minSoFar to Infinity?

Any real price will be smaller, so the first iteration correctly sets the minimum. Starting at 0 fails if all prices are positive.

What if I'm forced to make a transaction even if it loses money?

Different problem — return the smallest loss. But the prompt explicitly allows 0 profit (no transaction).

Practice these live with InterviewChamp.AI

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

Practice these live with InterviewChamp.AI →