123. Best Time to Buy and Sell Stock III (1)

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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.

Solution

 public int maxProfit(int[] prices) {
        int firstBuyCost = Integer.MAX_VALUE;
        int secondBuyCost = Integer.MAX_VALUE;
        int oneTradeProfit = 0;
        int twoTradeProfit = 0;
        for(int i = 0; i < prices.length; i++){
            firstBuyCost = Math.min(firstBuyCost, prices[i]);
            oneTradeProfit = Math.max(oneTradeProfit, prices[i]-firstBuyCost);
            //If we want to buy the stock in this run, 
            //we have to sell our first stock in hand
            secondBuyCost = Math.min(secondBuyCost, prices[i]-oneTradeProfit);
            twoTradeProfit = Math.max(twoTradeProfit, prices[i]-secondBuyCost);
            
        }
        return Math.max(oneTradeProfit, twoTradeProfit);
        
    }