Merge Two Sorted Arrays Without Extra Space in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of merging two sorted arrays without extra space

What is the output of the following C# code after merging two sorted arrays without using extra space?

DSA C#

int[] arr1 = {1, 5, 9, 10, 15, 20};
int[] arr2 = {2, 3, 8, 13};

int n = arr1.Length;
int m = arr2.Length;

for (int i = 0; i < n; i++) {
    if (arr1[i] > arr2[0]) {
        int temp = arr1[i];
        arr1[i] = arr2[0];
        arr2[0] = temp;

        // Move arr2[0] to correct position to keep arr2 sorted
        int first = arr2[0];
        int k;
        for (k = 1; k < m && arr2[k] < first; k++) {
            arr2[k - 1] = arr2[k];
        }
        arr2[k - 1] = first;
    }
}

Console.WriteLine(string.Join(", ", arr1));
Console.WriteLine(string.Join(", ", arr2));

A 1, 2, 3, 5, 8, 9\n10, 13, 15, 20

B 1, 3, 5, 8, 9, 10\n2, 13, 15, 20

C 1, 2, 3, 5, 9, 10\n8, 13, 15, 20

D 02 ,51 ,31 ,01n\9 ,8 ,5 ,3 ,2 ,1

💡 Hint

Focus on how the smallest element of the second array is swapped and repositioned to keep both arrays sorted.

The code swaps elements from the first array with the smallest element of the second array if the first array's element is bigger. Then it repositions the swapped element in the second array to maintain sorted order. This results in both arrays sorted without extra space.

🧠 Conceptual

intermediate

Understanding the key step in merging without extra space

In the merge process without extra space, what is the main reason for repositioning the first element of the second array after swapping?

A To increase the size of the first array

B To remove duplicates from both arrays

C To sort the first array again

D To maintain the sorted order of the second array

💡 Hint

Think about why the second array might become unsorted after swapping.

After swapping, the first element of the second array might be larger than some elements following it, so repositioning it keeps the second array sorted.

🔧 Debug

advanced

Identify the bug in this merge code snippet

What error will this code produce when merging two sorted arrays without extra space?

DSA C#

int[] arr1 = {1, 4, 7, 8, 10};
int[] arr2 = {2, 3, 9};

int n = arr1.Length;
int m = arr2.Length;

for (int i = 0; i < n; i++) {
    if (arr1[i] > arr2[0]) {
        int temp = arr1[i];
        arr1[i] = arr2[0];
        arr2[0] = temp;

        int first = arr2[0];
        int k = 1;
        while (k < m && arr2[k] < first) {
            arr2[k - 1] = arr2[k];
            k++;
        }
        arr2[k - 1] = first;
    }
}

Console.WriteLine(string.Join(", ", arr1));
Console.WriteLine(string.Join(", ", arr2));

A NullReferenceException

B No error, outputs merged arrays correctly

C Infinite loop

D IndexOutOfRangeException

💡 Hint

Check the loop conditions and array accesses carefully.

The code correctly repositions the swapped element in arr2 without exceeding array bounds, so no runtime error occurs.

❓ Predict Output

advanced

Result after merging two sorted arrays with duplicates

What is the output after merging these two sorted arrays without extra space?

DSA C#

int[] arr1 = {1, 3, 5, 7};
int[] arr2 = {0, 3, 6, 8};

int n = arr1.Length;
int m = arr2.Length;

for (int i = 0; i < n; i++) {
    if (arr1[i] > arr2[0]) {
        int temp = arr1[i];
        arr1[i] = arr2[0];
        arr2[0] = temp;

        int first = arr2[0];
        int k = 1;
        while (k < m && arr2[k] < first) {
            arr2[k - 1] = arr2[k];
            k++;
        }
        arr2[k - 1] = first;
    }
}

Console.WriteLine(string.Join(", ", arr1));
Console.WriteLine(string.Join(", ", arr2));

A 0, 1, 3, 3\n5, 6, 7, 8

B 0, 1, 3, 5\n3, 6, 7, 8

C 8 ,7 ,6 ,5n\3 ,3 ,1 ,0

D 8 ,7 ,6 ,3n\5 ,3 ,1 ,0

💡 Hint

Duplicates can appear in both arrays after merging.

The smallest elements are swapped and repositioned, resulting in arr1 = [0, 1, 3, 3] and arr2 = [5, 6, 7, 8].

🚀 Application

expert

Minimum number of swaps to merge two sorted arrays without extra space

Given two sorted arrays arr1 and arr2, what is the minimum number of swaps performed by the standard in-place merge algorithm (as shown in previous examples) to merge them without extra space? arr1 = [1, 4, 7, 8, 10] arr2 = [2, 3, 9]

A 3

B 5

C 4

D 2

💡 Hint

Count how many times an element from arr1 is swapped with arr2[0].

Swaps occur when arr1[i] > arr2[0]. For given arrays, swaps happen at i=1 (4>2), i=2 (7>3), i=3 (8>4), and i=4 (10>7), totaling 4 swaps.

Practice

1. What is the main goal when merging two sorted arrays without extra space?

easy

A. Create a new array to hold merged elements

B. Combine both arrays into one sorted sequence without using extra memory

C. Copy elements from one array to another without sorting

D. Sort each array separately using extra space

Solution

1. Step 1: Understand the problem requirement

   The task is to merge two sorted arrays so that the combined elements remain sorted, but without using any extra space like a new array.

2. Step 2: Identify the correct approach

   We must rearrange elements within the given arrays themselves to maintain sorted order without additional memory.

3. Final Answer:

   Combine both arrays into one sorted sequence without using extra memory -> Option B

4. Quick Check:

   Merge without extra space = Combine in place [OK]

Hint: Merge by swapping elements between arrays without extra space [OK]

Common Mistakes:

• Assuming extra space is allowed
• Sorting arrays separately without merging
• Creating a new array for merged result

2. Which of the following C# code snippets correctly swaps elements between two arrays without extra space?

easy

A. arr1[i] = arr1[i] + arr2[j]; arr2[j] = arr1[i] - arr2[j]; arr1[i] = arr1[i] - arr2[j];

B. arr1[i] = arr2[j]; arr2[j] = arr1[i];

C. if (arr1[i] > arr2[j]) { int temp = arr1[i]; arr1[i] = arr2[j]; arr2[j] = temp; }

D. int temp = arr2[j]; arr2[j] = arr1[i]; arr1[i] = temp + 1;

Solution

1. Step 1: Check swapping logic correctness

   if (arr1[i] > arr2[j]) { int temp = arr1[i]; arr1[i] = arr2[j]; arr2[j] = temp; } uses a temporary variable to swap values correctly without losing data.

2. Step 2: Analyze other options for errors

   arr1[i] = arr2[j]; arr2[j] = arr1[i]; overwrites arr2[j] before saving arr1[i], losing original value. arr1[i] = arr1[i] + arr2[j]; arr2[j] = arr1[i] - arr2[j]; arr1[i] = arr1[i] - arr2[j]; uses arithmetic swap but can cause overflow and is less safe. int temp = arr2[j]; arr2[j] = arr1[i]; arr1[i] = temp + 1; incorrectly adds 1 during swap, changing values.

3. Final Answer:

   if (arr1[i] > arr2[j]) { int temp = arr1[i]; arr1[i] = arr2[j]; arr2[j] = temp; } -> Option C

4. Quick Check:

   Temporary variable swap = Correct swap [OK]

Hint: Use a temp variable to swap safely in C# [OK]

Common Mistakes:

• Overwriting values without temp variable
• Using unsafe arithmetic swaps
• Altering values unintentionally during swap

3. What will be the content of arrays after running this code?

int[] arr1 = {1, 5, 9, 10, 15, 20};
int[] arr2 = {2, 3, 8, 13};
int n = arr1.Length, m = arr2.Length;
for (int i = 0; i < n; i++) {
  if (arr1[i] > arr2[0]) {
    int temp = arr1[i];
    arr1[i] = arr2[0];
    arr2[0] = temp;
    int first = arr2[0];
    int k;
    for (k = 1; k < m && arr2[k] < first; k++) {
      arr2[k - 1] = arr2[k];
    }
    arr2[k - 1] = first;
  }
}

medium

A. arr1 = [1, 2, 3, 5, 9, 10], arr2 = [8, 13, 15, 20]

B. arr1 = [1, 3, 5, 8, 9, 10], arr2 = [2, 13, 15, 20]

C. arr1 = [1, 2, 5, 8, 9, 10], arr2 = [3, 13, 15, 20]

D. arr1 = [1, 2, 3, 5, 8, 9], arr2 = [10, 13, 15, 20]

Solution

1. Step 1: Trace the swapping and insertion in arr2

   For each element in arr1, if it is greater than arr2[0], swap and then insert arr2[0] in sorted order in arr2. This pushes larger elements to arr2.

2. Step 2: Final arrays after all iterations

   After all swaps and insertions, arr1 becomes [1, 2, 3, 5, 8, 9] and arr2 becomes [10, 13, 15, 20].

3. Final Answer:

   arr1 = [1, 2, 3, 5, 8, 9], arr2 = [10, 13, 15, 20] -> Option D

4. Quick Check:

   In-place merge with insertion sort = arr1 and arr2 sorted [OK]

Hint: Swap if arr1[i] > arr2[0], then insert arr2[0] sorted [OK]

Common Mistakes:

• Not inserting arr2[0] correctly after swap
• Overwriting arr2 elements incorrectly
• Confusing array indices during insertion

4. Identify the error in this code snippet for merging two sorted arrays without extra space:

for (int i = 0; i < n; i++) {
  if (arr1[i] > arr2[0]) {
    int temp = arr1[i];
    arr1[i] = arr2[0];
    arr2[0] = temp;
    int first = arr2[0];
    int k = 1;
    while (k < m && arr2[k] < first) {
      arr2[k - 1] = arr2[k];
      k++;
    }
    arr2[k] = first;
  }
}

medium

A. The insertion index in arr2 is incorrect; should be arr2[k - 1] = first

B. The swap logic between arr1[i] and arr2[0] is wrong

C. The loop should run from i = 1 instead of 0

D. The condition arr2[k] < first should be arr2[k] > first

Solution

1. Step 1: Analyze insertion logic in arr2

   The code shifts elements left to make space for 'first'. The correct insertion index after shifting is k - 1, not k.

2. Step 2: Identify the error impact

   Assigning arr2[k] = first overwrites the next element incorrectly, causing wrong order.

3. Final Answer:

   The insertion index in arr2 is incorrect; should be arr2[k - 1] = first -> Option A

4. Quick Check:

   Correct insertion index = k - 1 [OK]

Hint: Insert shifted element at k-1 index after loop ends [OK]

Common Mistakes:

• Using wrong index for insertion after shifting
• Incorrect swap logic
• Wrong loop start index

5. Given two sorted arrays:
arr1 = [1, 4, 7, 8, 10]
arr2 = [2, 3, 9]
How would you merge them without extra space so that after merging, arr1 contains the first 5 smallest elements and arr2 contains the remaining sorted elements? Choose the correct final arrays.

hard

A. arr1 = [1, 2, 3, 4, 7], arr2 = [8, 9, 10]

B. arr1 = [1, 3, 4, 7, 8], arr2 = [2, 9, 10]

C. arr1 = [1, 2, 4, 7, 8], arr2 = [3, 9, 10]

D. arr1 = [1, 2, 3, 4, 8], arr2 = [7, 9, 10]

Solution

1. Step 1: Compare and swap elements between arr1 and arr2

   Start from beginning of arr1, if arr1[i] > arr2[0], swap and then insert arr2[0] in sorted order in arr2.

2. Step 2: After all swaps and insertions

   arr1 becomes [1, 2, 3, 4, 7] and arr2 becomes [8, 9, 10], maintaining sorted order without extra space.

3. Final Answer:

   arr1 = [1, 2, 3, 4, 7], arr2 = [8, 9, 10] -> Option A

4. Quick Check:

   In-place merge keeps first array smallest elements [OK]

Hint: Swap if arr1[i] > arr2[0], then insert arr2[0] sorted [OK]

Common Mistakes:

• Not maintaining sorted order after swap
• Incorrect insertion in second array
• Assuming extra space is allowed