[Leetcode] 121. Best Time to Buy and Sell Stock

題目 : 121. Best Time to Buy and Sell Stock

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 1:

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.

Example 2:

Input: prices = [7,6,4,3,1]
Output: 0
Explanation: In this case, no transactions are done and the max profit = 0.

參考影片 : https://youtu.be/1pkOgXD63yU

class Solution:
    def maxProfit(self, prices: List[int]) -> int:
        l, r = 0 ,1 # Left=buy, Right=sell
                    # 0,1 分別代表索引值
        maxP = 0    # 最高利潤值

        while r < len(prices):  
            if prices[l] < prices[r] :   # 如果左側l的金額<右側r的金額，代表可以買低賣高
                profit = prices[r] - prices[l]  # 計算利潤
                maxP = max(maxP, profit)   # 用max()回傳較大的數字到maxP，得到目前的最高利潤
            else:
                l = r 
            r += 1

        return maxP