Best_Time_to_Buy_and_Sell_Stock.md

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: 5

max. 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: 0

In this case, no transaction is done, i.e. max profit = 0.

Solution:

一次遍历求出后元素减前元素的最大值：

class Solution(object):
    def maxProfit(self, prices):
        """
        :type prices: List[int]
        :rtype: int
        """
        if len(prices)<2:
            return 0
        minprice=prices[0]
        maxprofit=0
        for price in prices:
            if price<minprice:
                minprice=price
            if price-minprice>maxprofit:
                maxprofit=price-minprice
        return maxprofit