122-Best Time to Buy and Sell Stock II

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题意

Say you have an array prices for which the ith element is the price of a given stock on day i. Design an algorithm to find the maximum profit. You may complete as many transactions as you like (i.e., buy one and sell one share of the stock multiple times). Note: You may not engage in multiple transactions at the same time (i.e., you must sell the stock before you buy again).

Example 1:
Input: [7,1,5,3,6,4]
Output: 7
Explanation: Buy on day 2 (price = 1) and sell on day 3 (price = 5), profit = 5-1 = 4.
             Then buy on day 4 (price = 3) and sell on day 5 (price = 6), profit = 6-3 = 3.

Example 2:
Input: [1,2,3,4,5]
Output: 4
Explanation: Buy on day 1 (price = 1) and sell on day 5 (price = 5), profit = 5-1 = 4.
             Note that you cannot buy on day 1, buy on day 2 and sell them later, as you are
             engaging multiple transactions at the same time. You must sell before buying again.

思路

• 只能一个交易进行中
• 贪心算法: 能赚一点十一点
• 只要当天相对于前一天上涨了 ->就前一天买入，当天卖出

class Solution(object):
    def maxProfit(self, prices):
        """
        :type prices: List[int]
        :rtype: int
        """
        # 只能一个交易进行中
        # 贪心算法
        # 只要当天相对于前一天上涨了 ->就前一天买入，当天卖出
        if not prices or len(prices) < 2: return 0
        profit = 0
        
        for i in range(1,len(prices)): # 注意从第二天开始比较
            if prices[i] > prices[i-1]:
                profit += prices[i] - prices[i-1]
        
        return profit

分析:

• Time: O(n)
• Space: O(1)