题目描述

You are given an array prices where prices[i] is the price of a given stock on the ith day.

Find the maximum profit you can achieve. 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 simultaneously (i.e., you must sell the stock before you buy again).

样例描述

Example 1:

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

Constraints:

1 <= prices.length <= 3 * 104
0 <= prices[i] <= 104

思路

1. 下次买入前必须卖出已有的股票，所以两次交易不能有数轴上的交集，如下
   
2. 一次买入卖出过程可以分解为单天的买入卖出，则这个问题就变成：在所有的单天里面找买入卖出是正利润的总和即可。
3. 由2可以先算所有的单天买入卖出，把正的利润加起来就是所求

代码

class Solution {
    public int maxProfit(int[] prices) {
       int res = 0;
       for (int i = 0; i + 1 < prices.length; i++){
          int t = prices[i + 1] - prices[i];
          if (t > 0) res += t;
       }
       return res;
    }
}