A Simple Merge Sort Implementation [C#]

MergeSort is a "divide and conquer" algorithm that splits an array into two halves (sub arrays) and recursively sorts each sub array before merging them back into one giant, sorted array.

 

In this blog, I will provide a simple implementation of MergeSort with comments on every significant line of code for beginners to quickly grasp the algorithm.

 

Pseudocode

mergeSort(array)

if array.length <= 1 then

return array 

left = new array

right = new array 

mid = left+ right/2

mergeSort(left)

mergeSort(right)

merge(left, right)  

class MergeSort
    {
        public static int[] mergeSort(int[] array)
        {
            int[] left;
            int[] right;
            int[] result = new int[array.Length];

            //As this is a recursive algorithm, we need to have a base case to
            //avoid an infinite recursion and therfore a stackoverflow
            if (array.Length <= 1)
                return array;

            // The exact midpoint of our array
            int midPoint = array.Length / 2;

            //Will represent our 'left' array
            left = new int[midPoint];

            //if array has an even number of elements, the left and right array will have the same number of
            //elements
            if (array.Length % 2 == 0)
                right = new int[midPoint];

            //if array has an odd number of elements, the right array will have one more element than left
            else
                right = new int[midPoint + 1];

            //populate left array
            for (int i = 0; i < midPoint; i++)
                left[i] = array[i];

            //populate right array
            int x = 0;
            //We start our index from the midpoint, as we have already populated the left array from 0 to
            midpont
            for (int i = midPoint; i < array.Length; i++)
            {
                right[x] = array[i];
                x++;
            }

            //Recursively sort the left array
            left = mergeSort(left);
            //Recursively sort the right array
            right = mergeSort(right);
            //Merge our two sorted arrays
            result = merge(left, right);

            return result;
        }

        //This method will be responsible for combining our two sorted arrays into one giant array
        public static int[] merge(int[] left, int[] right)
        {
            int resultLength = right.Length + left.Length;
            int[] result = new int[resultLength];
            //
            int indexLeft = 0, indexRight = 0, indexResult = 0;

            //while either array still has an element
            while (indexLeft < left.Length || indexRight < right.Length)
            {
                //if both arrays have elements
                if (indexLeft < left.Length && indexRight < right.Length)
                {
                    //If item on left array is less than item on right array, add that item to the result array
                    if (left[indexLeft] <= right[indexRight])
                    {
                        result[indexResult] = left[indexLeft];
                        indexLeft++;
                        indexResult++;
                    }
                    // else the item in the right array wll be added to the results array
                    else
                    {
                        result[indexResult] = right[indexRight];
                        indexRight++;
                        indexResult++;
                    }
                }
                //if only the left array still has elements, add all its items to the results array
                else if (indexLeft < left.Length)
                {
                    result[indexResult] = left[indexLeft];
                    indexLeft++;
                    indexResult++;
                }
                //if only the right array still has elements, add all its items to the results array
                else if (indexRight < right.Length)
                {
                    result[indexResult] = right[indexRight];
                    indexRight++;
                    indexResult++;
                }

            }
            return result;
        }
    }

You can now go ahead and call your MergeSort(array) method from Main to see the results.