Leetcode 122 Best time to buy and sell stockII

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博客围绕股票最大利润问题展开，给定数组表示每日股票价格，可进行多次交易但不能同时多笔交易。解题思路是找出数组单调递增区间，计算区间增加值，只要满足prices[i-1] < prices[i]，就将差值累加到利润中。

问题描述

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.

解题思路

我们想要找到最大的利润，不限制交易次数。

只要股票的价格是上升的，那么我们就能赚取利润，下降的时候。我们不做买卖。

说白了，我们就是找到整个数组的单调递增区间，计算整个区间的增加值即可

举例： 1 2 3 4 5 是一个单调递增的区间，最大利润为 5-1。

举例： 1,2，5,3,4 最大利润是（5-1 + 4-3）为5。其中125是一个递增序列，34是一个递增序列

举例：7,6,5,1,4,3,2 最大利润为 4-1 =3 其他阶段都是递减只要 14是递增。

条件就是 prices[i-1] < prices[i] :利润+= prices[i]-prices[i-1]

其实以1,2,3,4,5为例，首先是利润 += 2-1 然后发现3比2还大，然后就利润+= 3-2 然后发现4比3还大再利润+=4-3.... 这个过程 2-1+3-2+4-3 + 5-4 = 5-1 = 4

题解

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