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).

分析

首先不壓縮空間的寫法,寫法依然參照stock problem精帖的base case ,分別用一個二維陣列去儲存sell和buy的狀況

const int inf=-999999;
class Solution {
public:
    int maxProfit(vector<int>& prices) {
        if(prices.size()<=1)
           return 0;
        int days=prices.size();
        int sell[days+1][3],buy[days+1][3];
        for(int i=0;i<3;i++){
            sell[0][i]=0;
            buy[0][i]=inf;
        }
        for(int i=0;i<=prices.size();i++){
            sell[i][0]=0;
            buy[i][0]=inf;
        }
        for(int i=1;i<=prices.size();i++){
            for(int k=1;k<=2;k++){
                sell[i][k]=max(sell[i-1][k],buy[i-1][k]+prices[i-1]);
                buy[i][k]=max(buy[i-1][k],sell[i-1][k-1]-prices[i-1]);
            }
        }
        return sell[prices.size()][2];
    }
};