121. Best Time to Buy and Sell Stock

Say you have an array for which the ith element is the price of a given stock on day i.If you were only permitted to complete at most one transaction (ie, buy one and sell one share of the stock), design an algorithm to find the maximum profit.Example 1:Input: [7, 1, 5, 3, 6, 4]Output: 5max. difference = 6-1 = 5 (not 7-1 = 6, as selling price needs to be larger than buying price)Example 2:Input: [7, 6, 4, 3, 1]Output: 0In this case, no transaction is done, i.e. max profit = 0.

解：
找到已经遍历过的数值中的最小值，然后找到后面和这个值的差的最大值。然后找到差值的最大值。

class Solution(object):    def maxProfit(self, prices):        """        :type prices: List[int]        :rtype: int        """        minprice =float('Inf')        maxprofit = 0        for i in range(len(prices)):            minprice = min(minprice, prices[i])            maxprofit = max(maxprofit, prices[i] - minprice)        return maxprofit