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4. Best Time to Buy and Sell Stock

easyAsked at Autodesk

Find the max profit from one buy-and-sell of a stock given daily prices.

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. Return the maximum profit you can achieve by choosing a single day to buy and a later day to sell. 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 pairs

Try every (buy, sell) pair and track the max gain.

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. Running minimum

Track the minimum price seen so far and compare each new price against it for max profit. One linear pass.

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

Tradeoff:

Autodesk-specific tips

Autodesk likes single-pass min/max trackers because the same shape powers bounding-box reductions and ray casting min-t computations.

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Output

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