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

You are given an array prices where prices[i] is the price of a given stock on the ith day.
You want to maximize your profit by choosing a single day to buy one stock and choosing a different day in the future to sell that stock.
Return the maximum profit you can achieve from this transaction. If you cannot achieve any profit, return 0.

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

set purchase price to price from first day, profit to 0
loop every prices from the 2nd day,
compare purchase price and update purchase price to smaller price.
compare current profit to difference of current price and purchase price