[leetcode]Best Time to Buy and Sell Stock III

Say you have an array for which the i^th 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).

 

class Solution {
public:
    int maxProfit(vector<int> &prices) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(prices.empty()) return 0;
        int N = prices.size();
        
        vector<int> forward(N, 0);
        vector<int> backward(N, 0);
        
        int curmin = prices[0];
        int result = 0;
        
        for(int i = 1; i < prices.size(); i++){
            curmin = min(curmin, prices[i]);
            result = max(result, prices[i] - curmin);
            
            forward[i] = result;
        }
        
        int curmax = prices.back();
        result = 0;
        for(int i = N-2; i >= 0; i--){
            curmax = max(curmax, prices[i]);
            result = max(result, curmax - prices[i]);
            
            backward[i] = result;
        }
        
        int ret = 0;
        for(int i = 0; i < N-1; i++){
            ret = max(ret, forward[i] + backward[i+1]);
        }
        
        ret = max(ret, forward.back());
        
        return ret;
    }
};