leetcode 123. Best Time to Buy and Sell Stock III ----- java

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).

之前题的扩展，要求是只能买卖两次，然后求出最大利润。

利用动态规划，从前向后和从后向前各求一次，然后求出结果。

public class Solution {
    public int maxProfit(int[] prices) {
        int len = prices.length;
        if( len < 2 )
            return 0;
        int[] preProfit = new int[len];
        int[] maxProfit = new int[len];
        int cur = prices[0];
        preProfit[0] = 0;
        for( int i = 1;i<len;i++){
            cur = Math.min(cur,prices[i]);
            preProfit[i] = Math.max(preProfit[i-1],prices[i] - cur);
        }
        cur = prices[len-1];
        maxProfit[len-1] = 0;
        for( int i = len-2;i>=0;i--){
            cur = Math.max(cur,prices[i]);
            maxProfit[i] = Math.max(maxProfit[i+1],cur - prices[i]);
        }
        
        int result = 0;
        for( int i = 0;i<len;i++){
            result = Math.max(preProfit[i]+maxProfit[i],result);
        }
        return result;

    
    }
}