# [122. Best Time to Buy and Sell Stock II](https://leetcode.com/problems/best-time-to-buy-and-sell-stock-ii/)


## 题目

Say you have an array for which the *i*th 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.

## 题目大意

给定一个数组，它的第 i 个元素是一支给定股票第 i 天的价格。设计一个算法来计算你所能获取的最大利润。你可以尽可能地完成更多的交易（多次买卖一支股票）。注意：你不能同时参与多笔交易（你必须在再次购买前出售掉之前的股票）。


## 解题思路

- 这一题是第 121 题的加强版。要求输出最大收益，这一题不止买卖一次，可以买卖多次，买卖不能在同一天内操作。
- 最大收益来源，必然是每次跌了就买入，涨到顶峰的时候就抛出。只要有涨峰就开始计算赚的钱，连续涨可以用两两相减累加来计算，两两相减累加，相当于涨到波峰的最大值减去谷底的值。这一点看通以后，题目非常简单。