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16. Best Time to Buy and Sell Stock

easyAsked at Ola

Maximize profit from a single buy/sell of 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. You want to maximize your profit by choosing a single day to buy and a different later day to sell. Return the maximum profit; 0 if not 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 day with every later sell day.

Time
O(n^2)
Space
O(1)
let max = 0;
for (let i=0;i<prices.length;i++)
  for (let j=i+1;j<prices.length;j++)
    max = Math.max(max, prices[j]-prices[i]);
return max;

Tradeoff:

2. Single pass minimum

Track the lowest price so far and update the best profit each step.

Time
O(n)
Space
O(1)
function maxProfit(prices) {
  let min = Infinity, best = 0;
  for (const p of prices) {
    min = Math.min(min, p);
    best = Math.max(best, p - min);
  }
  return best;
}

Tradeoff:

Ola-specific tips

Ola sees this as the entry to streaming-aggregate questions; relate it to capturing the best surge-pricing window seen so far in a live fare feed.

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Output

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