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."
Algorithm :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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Previous: Write a C# Sharp program to sort a list of elements using Insertion sort.
Next: Write a C# Sharp program to sort a list of elements using Permutation sort.
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