題目：

Say you have an array 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. 
 

Example 3:

Input: [7,6,4,3,1] 
Output: 0 
Explanation: In this case, no transaction is done, i.e. max profit = 0.

解釋： 一共可以進行任意多次操作，但是在買入一個stock之前必須先賣出手中已有的stock，每天可以先賣出已有的stock再買入新的stock，求能獲得的最大利潤。 python程式碼：

class Solution(object):
    def maxProfit(self, prices):
        """
        :type prices: List[int]
        :rtype: int
        """
 

        return sum([max(prices[i+1]-prices[i],0) for i in xrange(len(prices)-1)])

c++程式碼：

class Solution {
public:
    int maxProfit(vector<int>& prices) {
        if(prices.size()==0)
            return 0;
        int _sum=0;
        for (int i=0;i<prices.size()-1;i++)
        {
            _sum+=max(0,prices[i+1]-prices[i]);
        }
        return _sum;
    }
};

總結： 注意題目所給條件。