leetcode 121.买卖股票的最佳时机

从前往后遍历：

class Solution:
    def maxProfit(self, prices: List[int]) -> int:
        if (len(prices)==1):
            return 0

        max_profit = 0
        min_price = float('inf')
        for i in range(len(prices)):
            if prices[i] < min_price:
                min_price = prices[i]
            if max_profit < (prices[i]-min_price):
                max_profit = prices[i]-min_price
        
        return max_profit