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16. Best Time to Buy and Sell Stock

easyAsked at Box

Maximize profit by choosing one buy day and one later sell day — Box uses this single-pass tracking pattern for cost/savings metrics on the storage dashboard.

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 day i. You want to maximize profit by choosing a single day to buy and a different day in the future to sell. Return the maximum profit, or 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 pairs

Check every (i, j) 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 minimum so far; profit = price - min. Single scan.

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 best = Math.max(best, p - min);
  }
  return best;
}

Tradeoff:

Box-specific tips

Box wants the running-minimum single-pass approach — they reuse this exact pattern for tracking lowest storage-cost watermarks across billing cycles.

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Output

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