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)
        {
            // Initialize lists for unsorted and sorted elements
            List<int> unsorted = new List<int>();
            List<int> sorted;

            // Create a random number generator
            Random random = new Random();

            Console.WriteLine("Original array elements:");
            // Generate and display random unsorted elements
            for(int i = 0; i < 10; i++)
            {
                unsorted.Add(random.Next(0, 100)); // Add a random number to the unsorted list
                Console.Write(unsorted[i] + " "); // Display the added number
            }
            Console.WriteLine();

            // Perform Merge Sort on the unsorted list
            sorted = MergeSort(unsorted);

            // Display the sorted array elements
            Console.WriteLine("Sorted array elements:");
            foreach (int x in sorted)
            {
                Console.Write(x + " ");
            }
			Console.Write("\n");
        }
		
        // Method to perform Merge Sort
        private static List<int> MergeSort(List<int> unsorted)
        {
            if (unsorted.Count <= 1)
                return unsorted; // Base case: return the list if it has only one element

            // Divide the unsorted list into two halves
            List<int> left = new List<int>();
            List<int> right = new List<int>();

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

            // Recursively perform Merge Sort on the divided lists
            left = MergeSort(left);
            right = MergeSort(right);

            // Merge the sorted halves
            return Merge(left, right);
        }

        // Method to merge two sorted lists
        private static List<int> Merge(List<int> left, List<int> right)
        {
            List<int> result = new List<int>();

            // Compare elements from both lists and merge them in sorted order
            while (left.Count > 0 || right.Count > 0)
            {
                if (left.Count > 0 && right.Count > 0)
                {
                    if (left.First() <= right.First()) // Compare the first elements of both lists
                    {
                        result.Add(left.First());
                        left.Remove(left.First()); // Remove the added element from the list
                    }
                    else
                    {
                        result.Add(right.First());
                        right.Remove(right.First()); // Remove the added element from the list
                    }
                }
                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; // Return the merged and sorted list
        }
    }
}

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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