LeetCode OJ 123 Best Time to Buy and Sell Stock III

题目：

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 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).

难度：hard

思路：正向扫面在i处左边可得的最大利润，逆向扫面在i处右边可得的最大利润，再将所得的两个数组相加求最大值

代码如下

class Solution {public:    int maxProfit(vector<int>& prices) {        vector<int> profit1(prices.size(),0);        vector<int> profit2(prices.size(),0);        if(prices.size()<=1)            return 0;        int minPrice=prices[0];        for(int i=1;i<prices.size();i++){            minPrice= min(minPrice,prices[i]);            profit1[i]=max(prices[i]-minPrice,profit1[i-1]);        }                int maxPrice=prices[prices.size()-1];        for(int i=prices.size()-2;i>=0;i--){            maxPrice = max(maxPrice,prices[i]);            profit2[i]=max(maxPrice-prices[i],profit2[i+1]);        }        int maxProfit=0;        for(int i=0;i<prices.size();i++){            maxProfit=max(maxProfit,profit1[i]+profit2[i]);        }        return maxProfit;    }};