C#: Merge sort

C# Sharp Searching and Sorting Algorithm: Exercise-7 with Solution

Write a C# Sharp program to sort a list of elements using Merge sort.

According to Wikipedia "Merge sort (also commonly spelled mergesort) is an O (n log n) comparison-based sorting algorithm. Most implementations produce a stable sort, which means that the implementation preserves the input order of equal elements in the sorted output."

Conceptually, a merge sort works as follows :

• Divide the unsorted list into n sublists, each containing 1 element (a list of 1 element is considered sorted).
• Repeatedly merge sublists to produce new sorted sublists until there is only 1 sublist remaining. This will be the sorted list.

An example of merge sort :

Merge Sort: Pictorial Presentation

Sample Solution:-

C# Sharp Code:

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

namespace Merge_sort
{    
    class Program
    {
        static void Main(string[] args)
        {
            List<int> unsorted = new List<int>();
            List<int> sorted;

            Random random = new Random();

            Console.WriteLine("Original array elements:" );
            for(int i = 0; i< 10;i++){
                unsorted.Add(random.Next(0,100));
                Console.Write(unsorted[i]+" ");
            }
            Console.WriteLine();

            sorted = MergeSort(unsorted);

            Console.WriteLine("Sorted array elements: ");
            foreach (int x in sorted)
            {
                Console.Write(x+" ");
            }
			Console.Write("\n");
        }
		

        private static List<int> MergeSort(List<int> unsorted)
        {
            if (unsorted.Count <= 1)
                return unsorted;

            List<int> left = new List<int>();
            List<int> right = new List<int>();

            int middle = unsorted.Count / 2;
            for (int i = 0; i < middle;i++)  //Dividing the unsorted list
            {
                left.Add(unsorted[i]);
            }
            for (int i = middle; i < unsorted.Count; i++)
            {
                right.Add(unsorted[i]);
            }

            left = MergeSort(left);
            right = MergeSort(right);
            return Merge(left, right);
        }

        private static List<int> Merge(List<int> left, List<int> right)
        {
            List<int> result = new List<int>();

            while(left.Count > 0 || right.Count>0)
            {
                if (left.Count > 0 && right.Count > 0)
                {
                    if (left.First() <= right.First())  //Comparing First two elements to see which is smaller
                    {
                        result.Add(left.First());
                        left.Remove(left.First());      //Rest of the list minus the first element
                    }
                    else
                    {
                        result.Add(right.First());
                        right.Remove(right.First());
                    }
                }
                else if(left.Count>0)
                {
                    result.Add(left.First());
                    left.Remove(left.First());
                }
                else if (right.Count > 0)
                {
                    result.Add(right.First());

                    right.Remove(right.First());    
                }    
            }
            return result;
        }
    }
}

Sample Output:

Original array elements:                                                                                      
79 69 9 95 65 49 65 40 27 95                                                                                  
Sorted array elements:                                                                                        
9 27 40 49 65 65 69 79 95 95

Flowchart:

C# Sharp Code Editor:

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