using System;

public class Program

{

    public static void Main()

    {

        var result = BuyAndSellStock(new int[]{21,12,11,9,6,3});

        Console.WriteLine(result);

    }

    public static int BuyAndSellStock(int[] arr)

    {

        if (arr == null || arr.Length == 0) return 0;

​

        int minBuy = arr[0], maxProfit = Int32.MinValue;

​

        for (int i = 1; i < arr.Length; i++)

        {

            int diff = arr[i] - minBuy;

            maxProfit = Math.Max(diff, maxProfit);

            minBuy = Math.Min(minBuy, arr[i]);      

        }

​

        return maxProfit;

    }

}