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.

Solution1:(TLE)

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

199 / 200 test cases passed.

Solution2:

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

一次遍历，只需要维护在这个点之前的最小值就可以。