leetcode 123.123. Best Time to Buy and Sell Stock III 困难

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 (i.e., you must sell the stock before you buy again).

Example 1:

Input: [3,3,5,0,0,3,1,4]
Output: 6
Explanation: Buy on day 4 (price = 0) and sell on day 6 (price = 3), profit = 3-0 = 3.

         Then buy on day 7 (price = 1) and sell on day 8 (price = 4), profit = 4-1 = 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.

class Solution {
public:
    int maxProfit(vector<int>& prices) {
        int len = prices.size();
        if(len == 0 ){
            return 0;
        }
        int firstbuy = 1<<30;
        int firstsell = - (1<<30);
        int secondbuy = - (1<<30);
        int secondsell = - (1<<30);
        for(int i = 0 ;i < len; ++i){
            firstbuy = min(firstbuy,prices[i]);
            firstsell = max(firstsell,prices[i]-firstbuy);
            secondbuy = max(secondbuy,firstsell - prices[i]);
            secondsell = max(secondsell,prices[i] + secondbuy);
        }
        return secondsell;
    }
};