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16. Best Time to Buy and Sell Stock

easyAsked at Zoom

Find the maximum profit from a single buy-and-sell of a stock.

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

Problem

Given an array prices where prices[i] is the price of a stock on day i, find the maximum profit from one buy and one later sell. If you cannot 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

Example 2

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

Approaches

1. Brute force

Try every (buy, sell) pair.

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

Tradeoff:

2. Running minimum

Track the minimum price seen and the best profit so far in one pass.

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

Tradeoff:

Zoom-specific tips

Zoom likes running-minimum patterns because they parallel jitter-buffer min/max tracking — explain the streaming analog as a bonus.

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Output

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