leetcode:买卖股票的最佳时机

class :
    def maxProfit(self, prices: List[int]) -> int:
        profit = 0
        if len(prices)==0:
            return 0
        low=prices[0]   
        for i in range(len(prices)):
            low = min(low, prices[i]);
            profit = max(profit, prices[i] - low);
            
        return  profit