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7. Best Time to Buy and Sell Stock

easyAsked at Baidu

Find the maximum profit from a single buy-then-sell over a daily price series.

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

Problem

Given an array prices where prices[i] is the stock price on day i, find the maximum profit you can achieve by buying on one day and selling on a later day. 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

Check every (buy, sell) pair and track the max difference.

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 with running min

Track the lowest price so far; at each day the candidate profit is today minus that min.

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

Tradeoff:

Baidu-specific tips

Baidu phrases this as 'maximum lift between two timestamps in an ad-ranking bid stream,' so they want the streaming single-pass and a clean comment tying min-so-far to the running anchor.

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Output

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