121. Best Time to Buy and Sell Stock

• keep the left pointer at the lowest position

class Solution:
    def maxProfit(self, prices: List[int]) -> int:
        
        l, r = 0, 1 
        curMax = 0

        while r < len(prices):
            if prices[r] - prices[l] >= 0:
                curMax = max(curMax, prices[r] - prices[l])
            else:
                l = r

            r += 1

        return curMax