Merge Sort Algorithm in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to split the array into two halves.

DSA C#

int mid = arr.Length [1] 2;

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A+

B/

C-

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using + or - instead of division to find the middle.

Using multiplication which increases the number.

The middle index is found by dividing the length of the array by 2.

2fill in blank

medium

Complete the code to copy the left half of the array.

DSA C#

int[] left = new int[mid];
Array.[1](arr, 0, left, 0, mid);

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ACloneTo

BClone

CCopyTo

DCopy

Attempts:

3 left

💡 Hint

Common Mistakes

Using Clone which returns an object, not copying elements.

Using CopyTo which is an instance method, not static.

Array.Copy copies elements from the source array to the destination array.

3fill in blank

hard

Fix the error in the merge function to compare elements correctly.

DSA C#

if (left[i] [1] right[j]) {
    arr[k++] = left[i++];
} else {
    arr[k++] = right[j++];
}

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A<

B>

C==

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using > which would reverse the order.

Using == or != which do not order elements.

We pick the smaller element first, so we use < to compare.

4fill in blank

hard

Fill both blanks to complete the recursive calls for merge sort.

DSA C#

MergeSort([1]);
MergeSort([2]);

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Aleft

Bright

Carr

Dmid

Attempts:

3 left

💡 Hint

Common Mistakes

Calling MergeSort on the whole array twice.

Using mid which is an index, not an array.

We recursively sort the left and right halves separately.

5fill in blank

hard

Fill all three blanks to complete the merge process indices.

DSA C#

int i = [1], j = [2], k = [3];
while (i < left.Length && j < right.Length) {
    if (left[i] < right[j]) {
        arr[k++] = left[i++];
    } else {
        arr[k++] = right[j++];
    }
}

Drag options to blanks, or click blank then click option'

A0

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Starting indices at 1 which skips the first element.

Using different values for i, j, k causing index errors.

Indices i, j, and k start at 0 to begin merging from the start of each array.

Practice

1. What is the main idea behind the Merge Sort algorithm?

easy

A. Pick the smallest element and place it at the beginning repeatedly

B. Divide the list into halves, sort each half, then merge them

C. Compare each element with every other element and swap if needed

D. Build a sorted list by inserting elements one by one

Solution

1. Step 1: Understand the divide and conquer approach

   Merge Sort splits the list into two halves recursively until each sublist has one element.

2. Step 2: Merge sorted halves

   Then it merges these small sorted lists back together in order to form a sorted list.

3. Final Answer:

   Divide the list into halves, sort each half, then merge them -> Option B

4. Quick Check:

   Merge Sort = Divide and conquer [OK]

Hint: Remember: split, sort halves, then merge [OK]

Common Mistakes:

• Confusing Merge Sort with Bubble Sort
• Thinking it sorts by swapping adjacent elements
• Believing it sorts by inserting elements one by one

2. Which of the following is the correct way to declare the merge function header in C# for Merge Sort?

easy

A. void Merge(int[] arr, int left, int mid, int right)

B. int Merge(int arr[], int left, int mid, int right)

C. void merge(int arr[], int start, int end)

D. int merge(int[] arr, int left, int right)

Solution

1. Step 1: Identify correct parameter types and names

   The merge function typically takes the array and three indices: left, mid, and right to merge two sorted halves.

2. Step 2: Check C# syntax for method declaration

   In C#, arrays are declared as int[] arr and the function returns void because it modifies the array in place.

3. Final Answer:

   void Merge(int[] arr, int left, int mid, int right) -> Option A

4. Quick Check:

   Correct signature = void Merge(int[] arr, int left, int mid, int right) [OK]

Hint: Look for void return and correct parameters [OK]

Common Mistakes:

• Using incorrect return type (int instead of void)
• Wrong parameter types or order
• Using lowercase function names in C#

3. What will be the output of the following C# code snippet using Merge Sort on array [4, 1, 3, 2]?

int[] arr = {4, 1, 3, 2};
MergeSort(arr, 0, arr.Length - 1);
Console.WriteLine(string.Join(", ", arr));

medium

A. 1, 3, 2, 4

B. 4, 3, 2, 1

C. 1, 2, 3, 4

D. 2, 1, 3, 4

Solution

1. Step 1: Apply Merge Sort to the array [4, 1, 3, 2]

   Split into [4,1] and [3,2], sort each half to [1,4] and [2,3].

2. Step 2: Merge the two sorted halves

   Merging [1,4] and [2,3] results in [1, 2, 3, 4].

3. Final Answer:

   1, 2, 3, 4 -> Option C

4. Quick Check:

   Sorted array = 1, 2, 3, 4 [OK]

Hint: Merge halves stepwise to get sorted array [OK]

Common Mistakes:

• Confusing merge order and producing unsorted output
• Not splitting array correctly
• Printing before sorting completes

4. Identify the bug in this Merge Sort merge function snippet:

void Merge(int[] arr, int left, int mid, int right) {
  int n1 = mid - left + 1;
  int n2 = right - mid;
  int[] L = new int[n1];
  int[] R = new int[n2];
  for (int i = 0; i < n1; i++) L[i] = arr[left + i];
  for (int j = 0; j < n2; j++) R[j] = arr[mid + 1 + j];
  // merging logic follows
}

medium

A. Incorrect array sizes for L and R

B. Loop conditions should use < instead of <= to avoid out of bounds

C. Wrong indices used for copying elements

D. Missing return statement in void function

Solution

1. Step 1: Check array size calculations

   The size of the left array should be mid - left + 1 to include the mid element; the right array size is correct as right - mid.

2. Step 2: Analyze loop conditions for copying elements

   Loops should use < to avoid out of bounds errors.

3. Final Answer:

   Incorrect array sizes for L and R -> Option A

4. Quick Check:

   Left array size must be mid - left + 1 [OK]

Hint: Array size for left = mid - left + 1 [OK]

Common Mistakes:

• Using <= causes index out of range errors
• Miscalculating array sizes
• Confusing mid index offsets

5. Given a large dataset with many duplicate values, how can you modify the Merge Sort algorithm to maintain stability (keep duplicates in original order)?

hard

A. Change Merge Sort to Quick Sort for better stability

B. Sort duplicates separately after Merge Sort completes

C. Use a hash set to remove duplicates before sorting

D. When merging, if elements are equal, take element from left subarray first

Solution

1. Step 1: Understand stability in sorting

   A stable sort keeps equal elements in the same order as original.

2. Step 2: Modify merge step to preserve order

   When merging, if elements are equal, always pick from the left subarray first to keep original order.

3. Final Answer:

   When merging, if elements are equal, take element from left subarray first -> Option D

4. Quick Check:

   Stable merge = pick left element first on ties [OK]

Hint: Pick left element first when equal during merge [OK]

Common Mistakes:

• Removing duplicates before sorting changes data
• Assuming Quick Sort is stable (it is not)
• Sorting duplicates separately after merge