Leetcode 122 - buy and sell stock - easy level algorithm - work on loops - check previous / next, play with boundary and duplicate numbers in a row.

Leetcode122_buyAndSellStock.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Leetcode122_buyAndSellStock
{
    /// <summary>
    /// Leetcode 122: 
    /// https://leetcode.com/problems/best-time-to-buy-and-sell-stock-ii/#/description
    /// </summary>
    class Program
    {
        static void Main(string[] args)
        {
        }

        public int MaxProfit(int[] prices)
        {
            if (prices == null || prices.Length <= 1)
            {
                return 0; 
            }

            var filtered = removeDuplicate(prices);
            if (filtered.Length <= 1)
            {
                return 0; 
            }

            // work on filtered array
            int length = filtered.Length;
            
            int buyPrice = 0;

            bool cycleStart = false;

            int profit = 0;            

            for (int i = 0; i < length; i++)
            {
                var current = filtered[i];
                bool isLastItem = i == (length - 1);

                bool isPeak1 = i == 0 && current > filtered[i + 1];
                bool isPeak2 = i > 0 && current > filtered[i - 1] &&
                    (isLastItem || current > filtered[i + 1]);

                if (cycleStart && ( isPeak1 || isPeak2 )) // skip first peak
                {
                    profit += current - buyPrice;
                    cycleStart = false; 
                }

                bool isBottom1 = i == 0 && current < filtered[i + 1];
                bool isBottom2 = i > 0  && current < filtered[i - 1]
                    && (i + 1 < length) && current < filtered[i + 1];

                if (!cycleStart && (isBottom1 || isBottom2))
                {
                    buyPrice = current;
                    cycleStart = true; 
                }
            }

            return profit; 
        }

        private static int[] removeDuplicate(int[] prices)
        {
            var filtered = new List<int>();

            for (int i = 0; i < prices.Length; i++)
            {
                if (i == 0 || prices[i] != prices[i - 1])
                {
                    filtered.Add(prices[i]);
                }
                else if (prices[i] == prices[i - 1])
                {
                    continue;
                }                
            }

            return filtered.ToArray();  
        }
    }
}