Array-6 Problem1: Buy and Sell Stock at Best Time (https://leetcode.com/problems/best-time-to-buy-and-sell-stock/) Problem2: Buy and Sell Stock at Best Time III (https://leetcode.com/problems/best-time-to-buy-and-sell-stock-iii/) Problem3: Buy and Sell Stock at Best Time IV (https://leetcode.com/problems/best-time-to-buy-and-sell-stock-iv/) Problem4: Buy and Sell Stock with cooldown https://leetcode.com/problems/best-time-to-buy-and-sell-stock-with-cooldown/