class Solution {

    public:

    int maxProfit(vector<int>& prices) {

    //It's wrong if you choose the minimum price for buy2 , but you can maximize the left money.

    //

    int buy1 = INT_MIN, sale1 = 0, buy2 = INT_MIN, sale2 = 0;

    for(int i=0; i<prices.size(); i++){                      //the more money left, the happier you will be

        buy1 = max(buy1, -prices[i]);                        //left money after buy1

        sale1 = max(sale1, prices[i] + buy1);                //left money after sale1

        buy2 = max(buy2, sale1 - prices[i]);                 //left money after buy2

        sale2 = max(sale2, prices[i] + buy2);                //left money after sale2

    }

    return sale2;

    }

};