Merge Two Sorted Arrays Without Extra Space in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to swap two integers using a temporary variable.

DSA C++

void swap(int &a, int &b) {
    int temp = [1];
    a = b;
    b = temp;
}

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

Atemp

Bb

Ca

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'b' instead of 'a' for the temporary variable.

Not using a temporary variable and losing data.

We store the value of 'a' in 'temp' before overwriting 'a' with 'b'. This preserves the original value of 'a' to assign to 'b'.

2fill in blank

medium

Complete the code to merge two sorted arrays without extra space by comparing elements.

DSA C++

for (int i = 0; i < n; i++) {
    if (arr1[i] > [1]) {
        swap(arr1[i], arr2[0]);
        // After swap, sort arr2 again
    }
}

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

Aarr1[n-1]

Barr2[n-1]

Carr1[0]

Darr2[0]

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing with the last element of arr2 instead of the first.

Comparing with elements of arr1 instead of arr2.

We compare arr1[i] with the first element of arr2 because arr2 is sorted, so the smallest element is at index 0.

3fill in blank

hard

Fix the error in the code that sorts the second array after swapping.

DSA C++

void sortSecondArray(int arr2[], int n) {
    for (int i = 1; i < n && arr2[i] < arr2[i - 1]; i++) {
        std::swap(arr2[i], arr2[[1]]);
    }
}

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

Ai - 1

Bi + 1

C0

Dn - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping with the next element instead of the previous.

Swapping with a fixed index like 0 or n-1.

We swap the current element arr2[i] with the previous element arr2[i - 1] to move the smaller element leftwards.

4fill in blank

hard

Fill both blanks to complete the function that merges two sorted arrays without extra space.

DSA C++

void merge(int arr1[], int arr2[], int n, int m) {
    for (int i = 0; i < n; i++) {
        if (arr1[i] > [1]) {
            std::swap(arr1[i], arr2[0]);
            int first = 0;
            int second = 1;
            while (second < m && arr2[second] < arr2[[2]]) {
                std::swap(arr2[first], arr2[second]);
                first++;
                second++;
            }
        }
    }
}

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

Aarr2[0]

Barr2[m-1]

C0

Dm-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong indices for comparison causing incorrect sorting.

Confusing array indices with array values.

We compare arr1[i] with the smallest element arr2[0]. Then, in the while loop, we compare arr2[second] with arr2[0] to bubble the swapped element to correct position.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps elements to their counts if count is greater than zero.

DSA C++

std::map<int, int> countMap;
// Assume countMap is filled with counts
std::map<int, int> filteredMap = {
    [1], [2]: [3] 
    for (auto const& [key, val] : countMap) if (val > 0)
};

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

Akey

Bval

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping key and val positions.

Using incorrect syntax for map comprehension.

We map key to val for all entries where val > 0. The comprehension syntax uses key and val accordingly.

Practice

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

easy

A. Copy elements from one array to another without sorting

B. Sort each array separately using extra space

C. Create a new array to hold merged elements

D. Combine both arrays into one sorted array without using extra memory

Solution

1. Step 1: Understand the problem requirement

   The problem asks to merge two sorted arrays into a single sorted sequence without using extra space, meaning no additional arrays or large buffers.

2. Step 2: Identify the correct approach

   We must rearrange elements within the given arrays to maintain sorted order combined, without allocating new memory.

3. Final Answer:

   Combine both arrays into one sorted array without using extra memory -> Option D

4. Quick Check:

   Merge without extra space = Combine in-place [OK]

Hint: Think in-place rearrangement, no new arrays needed [OK]

Common Mistakes:

• Assuming extra space is allowed
• Trying to merge by creating a new array
• Sorting arrays separately without merging

2. Which of the following C++ code snippets correctly swaps two elements without using extra space?

easy

A. a = a + b; b = a - b; a = a - b;

B. temp = a; a = b; b = temp;

C. a = b; b = a;

D. a = a * b; b = a / b; a = a / b;

Solution

1. Step 1: Analyze swapping without extra variable

   a = a + b; b = a - b; a = a - b; uses arithmetic operations to swap values without a temporary variable.

2. Step 2: Check correctness of each option

   temp = a; a = b; b = temp; uses a temporary variable, so it uses extra space. a = b; b = a; assigns a to b and then b to a, which loses original value. a = a * b; b = a / b; a = a / b; uses multiplication/division which can cause overflow or errors if b is zero.

3. Final Answer:

   a = a + b; b = a - b; a = a - b; -> Option A

4. Quick Check:

   Swap without temp = arithmetic trick [OK]

Hint: Use arithmetic swap to avoid extra variable [OK]

Common Mistakes:

• Using a temporary variable (extra space)
• Overwriting values without saving
• Using multiplication/division causing errors

3. What will be the content of arrays arr1 and arr2 after merging without extra space using the gap method?

arr1 = {1, 5, 9, 10}
arr2 = {2, 3, 8, 13}

medium

A. arr1 = {1, 2, 3, 5}, arr2 = {8, 9, 10, 13}

B. arr1 = {1, 3, 5, 8}, arr2 = {2, 9, 10, 13}

C. arr1 = {1, 2, 5, 8}, arr2 = {3, 9, 10, 13}

D. arr1 = {1, 2, 3, 8}, arr2 = {5, 9, 10, 13}

Solution

1. Step 1: Apply gap method to merge arrays

   Start with gap = ceil((4+4)/2) = 4. Compare elements at gap distance and swap if needed, then reduce gap until 0.

2. Step 2: Final sorted arrays after gap method

   After applying the method, arr1 becomes {1, 2, 3, 5} and arr2 becomes {8, 9, 10, 13}.

3. Final Answer:

   arr1 = {1, 2, 3, 5}, arr2 = {8, 9, 10, 13} -> Option A

4. Quick Check:

   Gap method merges arrays in-place correctly [OK]

Hint: Use gap method: reduce gap, swap if needed [OK]

Common Mistakes:

• Not reducing gap properly
• Swapping wrong elements
• Confusing array indices across two arrays

4. Identify the error in the following code snippet that attempts to merge two sorted arrays without extra space:

for (int i = 0; i < n; i++) {
  if (arr1[i] > arr2[0]) {
    swap(arr1[i], arr2[0]);
    sort(arr2, arr2 + m);
  }
}

medium

A. Swapping elements without checking bounds causes runtime error

B. Loop should run till m instead of n

C. Sorting arr2 inside the loop causes high time complexity

D. No error, code is optimal

Solution

1. Step 1: Analyze the loop and sorting inside it

   The code sorts arr2 inside the loop for every swap, which is inefficient and increases time complexity to O(n*m*log m).

2. Step 2: Identify the performance issue

   Sorting arr2 repeatedly is costly; better methods avoid sorting inside the loop.

3. Final Answer:

   Sorting arr2 inside the loop causes high time complexity -> Option C

4. Quick Check:

   Sorting inside loop = inefficient [OK]

Hint: Avoid sorting inside loops for better performance [OK]

Common Mistakes:

• Ignoring time complexity impact
• Assuming swap causes runtime error
• Confusing loop bounds

5. Given two sorted arrays arr1 = {10, 12, 15, 18} and arr2 = {2, 5, 20, 25}, how can you merge them without extra space so that after merging, arr1 contains the first four smallest elements and arr2 contains the remaining elements sorted?

hard

A. Swap elements only if arr1[i] is greater than arr2[j] without sorting arr2

B. Use the gap method to compare and swap elements across arrays until fully merged

C. Sort both arrays separately and then concatenate

D. Copy all elements to a new array, sort it, then copy back

Solution

1. Step 1: Understand the gap method for merging

   The gap method reduces the gap between compared elements and swaps if needed, working across both arrays without extra space.

2. Step 2: Apply gap method to given arrays

   By repeatedly comparing and swapping elements at the gap distance, arr1 will hold the smallest elements and arr2 the larger ones, both sorted.

3. Final Answer:

   Use the gap method to compare and swap elements across arrays until fully merged -> Option B

4. Quick Check:

   Gap method merges sorted arrays in-place [OK]

Hint: Use gap method for in-place merge without extra space [OK]

Common Mistakes:

• Using extra arrays violating constraints
• Sorting arrays separately without merging
• Swapping without sorting arr2 leads to wrong order