Implementing quicksort algorithm

Question

I found quicksort algorithm from this book

This is the algorithm

QUICKSORT (A, p, r)
i         


        

Answer 1

A simple generic C# implementation of QuickSort, can use first or last value or any other intermediate value for pivot

using System;

namespace QuickSort
{
    class Program
    {
        static void Main(string[] args)
        {
            int[] arInt = { 6, 4, 2, 8, 4, 5, 4, 5, 4, 5, 4, 8, 11, 1, 7, 4, 13, 5, 45, -1, 0, -7, 56, 10, 57, 56, 57, 56 };
            GenericQuickSort.QuickSort(arInt, 0, arInt.Length - 1);

            string[] arStr = { "Here", "Is", "A", "Cat", "Really", "Fast", "And", "Clever" };
            GenericQuickSort.QuickSort(arStr, 0, arStr.Length - 1); ;

            Console.WriteLine(String.Join(',', arInt));
            Console.WriteLine(String.Join(',', arStr));

            Console.ReadLine();
        }

    }

    class GenericQuickSort where T : IComparable
    {

        public static void QuickSort(T[] ar, int lBound, int uBound)
        {
            if (lBound < uBound)
            {
                var loc = Partition(ar, lBound, uBound);
                QuickSort(ar, lBound, loc - 1);
                QuickSort(ar, loc + 1, uBound);
            }
        }

        private static int Partition(T[] ar, int lBound, int uBound)
        {
            var start = lBound;
            var end = uBound;

            var pivot = ar[uBound];

            // switch to first value as pivot
            // var pivot = ar[lBound];

            while (start < end)
            {

                while (ar[start].CompareTo(pivot) < 0)
                {
                    start++;
                }

                while (ar[end].CompareTo(pivot) > 0)
                {
                    end--;
                }

                if (start < end)
                {
                    if (ar[start].CompareTo(ar[end]) == 0)
                    {
                        start++;
                    }
                    else
                    {
                        swap(ar, start, end);
                    }
                }
            }

            return end;
        }

        private static void swap(T[] ar, int i, int j)
        {
            var temp = ar[i];
            ar[i] = ar[j];
            ar[j] = temp;
        }
    }
}

Output:

-7,-1,0,1,2,4,4,4,4,4,4,5,5,5,5,6,7,8,8,10,11,13,45,56,56,56,57,57

A,And,Cat,Clever,Fast,Here,Is,Really

One important thing to notice here is that this optimized and simple code properly handles duplicates. I tried several posted quick sort code. Those do not give correct result for this (integer array) input or just hangs, such as https://www.w3resource.com/csharp-exercises/searching-and-sorting-algorithm/searching-and-sorting-algorithm-exercise-9.php and http://www.softwareandfinance.com/CSharp/QuickSort_Iterative.html. Therefore, if author also wants to use the code which handles duplicates this would be a good reference.