MergeSort.cs

using System;

namespace SortingAlgorithms
{
    static partial class Program
    {
        static void Merge(int[] arr, int l, int m, int r)
        {
            int n1 = m - l + 1;
            int n2 = r - m;

            int[] L = new int[n1];
            int[] R = new int[n2];

            for (int ii = 0; ii < n1; ii++)
            {
                L[ii] = arr[l + ii];
            }

            for (int jj = 0; jj < n2; jj++)
            {
                R[jj] = arr[m + 1 + jj];
            }

            int i = 0, j = 0, k = l;

            while (i < n1 && j < n2)
            {
                if (L[i] <= R[j])
                {
                    arr[k] = L[i];
                    i++;
                }
                else
                {
                    arr[k] = R[j];
                    j++;
                }
                k++;
            }

            while (i < n1)
            {
                arr[k] = L[i];
                i++;
                k++;
            }

            while (j < n2)
            {
                arr[k] = R[j];
                j++;
                k++;
            }
        }

        static void MergeSort(int[] arr, int l, int r)
        {
            if (l < r)
            {
                int m = (l + r) / 2;
                MergeSort(arr, l, m);
                MergeSort(arr, m + 1, r);
                Merge(arr, l, m, r);
            }

        }
    }
}