动态规划_leetcode121(好难，双状态)

class Solution(object):
    def maxProfit(self, prices):
        """
        :type prices: List[int]
        :rtype: int
        """

        if not prices:
            return 0


        buy = -prices[0]

        sell = 0


        for i in range(1,len(prices)):
            buy = max(buy ,-prices[i])
            sell = max (sell,prices[i] + buy)

        return sell