LC121 - Best Time to Buy and Sell Stock

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 one stock and choosing a different day in the future to sell that stock.

Return the maximum profit you can achieve from this transaction. If you cannot achieve any profit, return 0.

Example

Input: prices = [7,1,5,3,6,4]

Output: 5

Explanation: Buy on day 2 (price = 1) and sell on day 5 (price = 6), profit = 6-1 = 5. Note that buying on day 2 and selling on day 1 is not allowed because you must buy before you sell.

Solution

Time complexity is $O(n)$ and no extra space is used.

def maxProfit(self, prices: List[int]) -> int:
	buyPrice = prices[0]
	profit = 0

	for price in prices[1:]:
		if buyPrice > p:
			buyPrice = p
		
		profit = max(profit, p - buyPrice)
	
	return profit