121. Best Time to Buy and Sell Stock

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博客围绕股票买卖问题展开，给定数组表示每天股票价格，要求设计算法在最多进行一次交易（先买后卖）的情况下找出最大利润，还给出了相关示例。

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https://leetcode.com/problems/best-time-to-buy-and-sell-stock/description/

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 (i.e., buy one and sell one share of the stock), design an algorithm to find the maximum profit.

Note that you cannot sell a stock before you buy one.

Example 1:

Input: [7,1,5,3,6,4]
Output: 5
Explanation: Buy on day 2 (price = 1) and sell on day 5 (price = 6), profit = 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
Explanation: In this case, no transaction is done, i.e. max profit = 0.

class Solution(object):
    def maxProfit(self, prices):
        """
        :type prices: List[int]
        :rtype: int
        """
        length = len(prices)
        if length == 0:
            return 0
        left_min, right_max = prices[:], prices[:]
        for i in range(1, length):
            left_min[i] = min(left_min[i - 1], left_min[i])
        for i in range(length - 1, 0, -1):
            right_max[i - 1] = max(right_max[i - 1], right_max[i])
            
        profit = max([right_max[i] - left_min[i] for i in range(length)])
        return max(profit, 0)