Best Time to Buy and Sell Stock

class Solution(object):
    def maxProfit(self, prices):
        """
        :type prices: List[int]
        :rtype: int
        """
        l,r = 0,1
        max_profit = 0
        while r<len(prices):
            if prices[r]>prices[l]:
                max_profit = max(prices[r]-prices[l], max_profit)
            else:
                l = r
            r += 1
        return max_profit