[leetcode: Python] 121. Best Time to Buy and Sell Stock

题目：
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.

方法一：性能52ms

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

方法二：性能39ms

class Solution(object):    def maxProfit(self, prices):        """        :type prices: List[int]        :rtype: int        """        if len(prices) < 2: return 0        buy = prices[0]        sale = 0        profit = 0        for i in xrange(len(prices)):            if prices[i] < buy:                 buy = prices[i]            elif prices[i] > buy:                 sale = prices[i]                  if sale - buy > profit:                    profit = sale - buy        if sale == 0: return 0        return profit

方法一比较简洁。