BestTimeToBuyAndSellStock2.java

package algorithms;

/**
 * 122. Best Time to Buy and Sell Stock II
 * https://leetcode.com/problems/best-time-to-buy-and-sell-stock-ii/
 * Difficulty : Easy
 * Related Topics : Array, Greedy
 *
 * Say you have an array prices 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 as many transactions as you like (i.e., buy one and sell one share of the stock multiple times).
 *
 * 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: [7,1,5,3,6,4]
 * Output: 7
 * Explanation: Buy on day 2 (price = 1) and sell on day 3 (price = 5), profit = 5-1 = 4.
 *              Then buy on day 4 (price = 3) and sell on day 5 (price = 6), profit = 6-3 = 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.
 *
 *
 * Constraints:
 *
 * 1 <= prices.length <= 3 * 10 ^ 4
 * 0 <= prices[i] <= 10 ^ 4
 *
 * created by Cenk Canarslan on 2021-01-16
 */
public class BestTimeToBuyAndSellStock2 {

    /**
     * O(n)
     * @param prices
     * @return
     */
    public int maxProfit(int[] prices) {
        if ( ! validLength(prices)) {
            return -1;
        }
        int profit = 0;
        for (int i = 0; i < prices.length-1; i++) {
            if(prices[i+1] > prices[i]) {
                profit += prices[i+1] - prices[i];
            }
        }
        return profit;
    }

//    /**
//     * O(n)
//     * @param prices
//     * @return
//     */
//    public int maxProfit(int[] prices) {
//        if ( ! validLength(prices)) {
//            return -1;
//        }
//        int profit = 0;
//        for (int i = 1; i < prices.length; i++) {
//            int tmp_profit = prices[i] - prices[i-1];
//            if (tmp_profit > 0) {
//                profit += tmp_profit;
//            }
//        }
//        return profit;
//    }

    public static void main(String[] args) {
        BestTimeToBuyAndSellStock2 bestTime = new BestTimeToBuyAndSellStock2();
        System.out.println(bestTime.maxProfit(new int[]{7, 1, 5, 3, 6, 4, 7})); // 10
        System.out.println(bestTime.maxProfit(new int[]{7, 1, 5, 3, 6, 4})); // 7
        System.out.println(bestTime.maxProfit(new int[]{1, 2, 3, 4, 5})); // 4
        System.out.println(bestTime.maxProfit(new int[]{7, 6, 4, 3, 1})); // 0
    }

    private boolean validLength(int[] prices) {
        if (prices.length < 1 || prices.length > 3 * Math.pow(10d, 4d)) {
            return false;
        }
        return true;
    }
}