123. Best Time to Buy and Sell Stock III

1. 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 at most two transactions.

   Note:
   You may not engage in multiple transactions at the same time (ie, you must sell the stock before you buy again).

2. 解决思路
   和第121题一样的，只是变成了两次操作而已。用两个buy变量和两个sell变量实现动态规划即可。（注意，第二次buy的状态转移方程为buy2 = max(buy2,-prices[i]+sell1)）

3. 代码

class Solution {public:    int maxProfit(vector<int>& prices) {        if (prices.size() == 0)            return 0;        int buy1 = INT_MIN ,sell1 = INT_MIN , buy2 = INT_MIN, sell2 = INT_MIN;        for (int i = 0 ; i < prices.size(); ++i) {            buy1 = max(buy1,-prices[i]);            sell1 = max(sell1,prices[i]+buy1);            buy2 = max(buy2,sell1-prices[i]);            sell2 = max(sell2,buy2+prices[i]);        }        return sell2;    }};