C# Coding Test Course, Merge Sort

At this point where coding tests are gaining more and more popularity, algorithm problems are a topic that must be thoroughly understood. In this article, we will dive deep into the Merge Sort algorithm. Merge Sort is one of the sorting algorithms that generally performs well and is based on the ‘Divide and Conquer’ strategy. This article will explain the principles of Merge Sort, its implementation, and how to utilize Merge Sort through practical problems.

1. Overview of Merge Sort

Merge Sort works by splitting the list, sorting each part, and then merging them. It can be effectively used in various situations where the advantages of data structures and time complexity need to be considered. Merge Sort has a time complexity of O(n log n) in average, worst, and best cases, and it is a stable sorting algorithm. This means it maintains the relative order of data with the same value.

1.1. How Merge Sort Works

Merge Sort proceeds through the following steps:

  1. Divide the list in half and recursively sort each sublist.
  2. Merge the sorted sublists to create a single sorted list.

This process continues until the length of the list is 1, after which it is rearranged back together. The diagram below shows the basic flow of Merge Sort:

2. Implementation of Merge Sort

Let’s implement Merge Sort in C#. The basic implementation is as follows:

using System;

class MergeSort
{
    // Main method
    static void Main(string[] args)
    {
        int[] array = {38, 27, 43, 3, 9, 82, 10};

        Console.WriteLine("Before sorting: " + string.Join(", ", array));
        MergeSortAlgorithm(array, 0, array.Length - 1);
        Console.WriteLine("After sorting: " + string.Join(", ", array));
    }

    // Merge Sort algorithm
    static void MergeSortAlgorithm(int[] array, int left, int right)
    {
        if (left < right)
        {
            int mid = (left + right) / 2;

            // Division
            MergeSortAlgorithm(array, left, mid);
            MergeSortAlgorithm(array, mid + 1, right);

            // Merging
            Merge(array, left, mid, right);
        }
    }

    // Merge method
    static void Merge(int[] array, int left, int mid, int right)
    {
        // Create two sub-arrays
        int[] leftArray = new int[mid - left + 1];
        int[] rightArray = new int[right - mid];

        for (int i = 0; i < leftArray.Length; i++)
            leftArray[i] = array[left + i];

        for (int j = 0; j < rightArray.Length; j++)
            rightArray[j] = array[mid + 1 + j];

        int k = left;
        int a = 0;
        int b = 0;

        // Merging the arrays
        while (a < leftArray.Length && b < rightArray.Length)
        {
            if (leftArray[a] <= rightArray[b])
            {
                array[k] = leftArray[a];
                a++;
            }
            else
            {
                array[k] = rightArray[b];
                b++;
            }
            k++;
        }

        // Merging remaining elements
        while (a < leftArray.Length)
        {
            array[k] = leftArray[a];
            a++;
            k++;
        }

        while (b < rightArray.Length)
        {
            array[k] = rightArray[b];
            b++;
            k++;
        }
    }
}

2.1. Code Explanation

  • MergeSortAlgorithm: This method divides the array in half and sorts it through recursive calls. After sorting the left and right sub-arrays, it calls the Merge method to merge them.
  • Merge: This method receives two sub-arrays and merges them into a final sorted array. It uses pointers to compare the elements of the two sub-arrays and adds the smaller value to the final array.

3. Problem Example

Now let’s consider a simple problem where we can apply the Merge Sort algorithm.

Problem: Sort an Integer List

Write a method to sort the given list of integers in ascending order. The input consists of an array of integer values between 1 and 1,000,000, and can have up to 1,000 elements. Solve the problem using Merge Sort.

Input Example

{8, 3, 1, 7, 0, 10, 2}

Output Example

{0, 1, 2, 3, 7, 8, 10}

3.1. Problem Solving Process

To solve the above problem using the Merge Sort algorithm, we can use the previously written Merge Sort algorithm as is. Filling in the code here and testing with multiple input values will also be helpful.

using System;

class SortingExample
{
    static void Main(string[] args)
    {
        int[] array = {8, 3, 1, 7, 0, 10, 2};
        MergeSort(array);
        Console.WriteLine("Sorted array: " + string.Join(", ", array));
    }

    // Method to call Merge Sort
    static void MergeSort(int[] array)
    {
        MergeSortAlgorithm(array, 0, array.Length - 1);
    }

    // [Add the MergeSortAlgorithm and Merge methods here]
}

With the methods written like this, we can sort the given list of integers in ascending order. It is advisable to verify the algorithm’s validity through various tests with multiple input values.

4. Advantages and Disadvantages of Merge Sort

4.1. Advantages

  • Stable sorting: Data with the same value is sorted while maintaining its original order.
  • Predictable performance: Guarantees O(n log n) complexity even in the worst case.
  • Handling large datasets: Consistent performance with large datasets and easy implementation for external sorting.

4.2. Disadvantages

  • Additional space requirements: Requires additional memory for sorting.
  • Inefficient for small data: Simple sorting methods may be faster for small datasets.

5. Conclusion

In this post, we explored the Merge Sort algorithm in detail. We looked at how it can be utilized in the actual problem-solving process through specific implementations. Merge Sort can effectively serve for efficient and stable sorting, making it a very useful tool for solving various algorithm problems. In the next post, we will cover various algorithms as well. Thank you!