Array Reversal Techniques in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Array Reversal Using Two-Pointer Swap

What is the output of the following C code that reverses an array using two pointers?

DSA C

#include <stdio.h>

void reverse(int arr[], int n) {
    int start = 0, end = n - 1;
    while (start < end) {
        int temp = arr[start];
        arr[start] = arr[end];
        arr[end] = temp;
        start++;
        end--;
    }
}

int main() {
    int arr[] = {1, 2, 3, 4, 5};
    int n = 5;
    reverse(arr, n);
    for (int i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
    return 0;
}

A 1 2 3 4 5

B 1 5 3 2 4

C 5 4 3 2 1

D 5 2 3 4 1

💡 Hint

Think about how swapping elements from start and end moves towards the center.

The code swaps elements from the start and end moving inward, effectively reversing the array.

After reversal, the array elements print as: 5 4 3 2 1

❓ Predict Output

intermediate

Output of Array Reversal Using Recursion

What is the output of this C program that reverses an array recursively?

DSA C

#include <stdio.h>

void reverse(int arr[], int start, int end) {
    if (start >= end) return;
    int temp = arr[start];
    arr[start] = arr[end];
    arr[end] = temp;
    reverse(arr, start + 1, end - 1);
}

int main() {
    int arr[] = {10, 20, 30, 40};
    int n = 4;
    reverse(arr, 0, n - 1);
    for (int i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
    return 0;
}

A 10 20 30 40

B 40 30 20 10

C 40 20 30 10

D 10 40 30 20

💡 Hint

Recursion swaps outer elements and moves inward.

The recursive function swaps the first and last elements, then calls itself for the inner subarray.

Resulting reversed array: 40 30 20 10

🔧 Debug

advanced

Identify the Error in Array Reversal Code

What error does the following C code produce when trying to reverse an array?

DSA C

#include <stdio.h>

void reverse(int arr[], int n) {
    int i;
    for (i = 0; i <= n / 2; i++) {
        int temp = arr[i];
        arr[i] = arr[n - i];
        arr[n - i] = temp;
    }
}

int main() {
    int arr[] = {1, 2, 3, 4};
    int n = 4;
    reverse(arr, n);
    for (int i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
    return 0;
}

A Runtime error: Array index out of bounds

B No error, prints reversed array correctly

C Compilation error: missing semicolon

D Logical error: array remains unchanged

💡 Hint

Check the index used for swapping elements carefully.

The code accesses arr[n - i], but valid indices go up to n-1.

When i=0, arr[n - 0] = arr[4] is out of bounds causing runtime error.

❓ Predict Output

advanced

Output of Array Reversal Using XOR Swap

What is the output of this C code that reverses an array using XOR swap technique?

DSA C

#include <stdio.h>

void reverse(int arr[], int n) {
    int start = 0, end = n - 1;
    while (start < end) {
        arr[start] ^= arr[end];
        arr[end] ^= arr[start];
        arr[start] ^= arr[end];
        start++;
        end--;
    }
}

int main() {
    int arr[] = {7, 8, 9};
    int n = 3;
    reverse(arr, n);
    for (int i = 0; i < n; i++) {
        printf("%d ", arr[i]);
    }
    return 0;
}

A 9 7 8

B 8 7 9

C 7 8 9

D 9 8 7

💡 Hint

XOR swap exchanges values without a temporary variable.

The XOR swap swaps elements at start and end indices without extra space.

After reversal, array prints as: 9 8 7

🧠 Conceptual

expert

Time Complexity of Different Array Reversal Techniques

Which statement correctly describes the time complexity of common array reversal methods (two-pointer swap, recursion, XOR swap)?

A All methods have O(n) time complexity because each element is visited once.

B Recursion method has O(n^2) time complexity due to repeated calls.

C XOR swap method has O(n^2) time complexity because of bitwise operations.

D Two-pointer swap method has O(log n) time complexity as it swaps pairs.

💡 Hint

Consider how many times each element is accessed or swapped.

All three methods visit each element once or swap pairs once, resulting in linear time O(n).

Recursion and XOR swap do not increase complexity beyond O(n).

Practice

1. What does reversing an array mean in simple terms?

easy

A. Changing the order of elements so the last becomes first and first becomes last

B. Sorting the array in ascending order

C. Removing duplicate elements from the array

D. Adding new elements at the end of the array

Solution

1. Step 1: Understand array order

   An array has elements arranged from first to last index.

2. Step 2: Define reversal

   Reversing means swapping elements so the last element moves to the first position and vice versa.

3. Final Answer:

   Changing the order of elements so the last becomes first and first becomes last -> Option A

4. Quick Check:

   Reversal = Reverse order [OK]

Hint: Reversal flips the array order front to back [OK]

Common Mistakes:

• Confusing reversal with sorting
• Thinking reversal removes elements
• Assuming reversal adds elements

2. Which of the following C code snippets correctly swaps two elements arr[i] and arr[j] for array reversal?

easy

A. arr[i] = arr[j]; arr[j] = arr[i];

B. temp = arr[j]; arr[j] = arr[i]; arr[i] = temp;

C. int temp = arr[i]; arr[i] = arr[j]; arr[j] = temp;

D. arr[i] = arr[j]; arr[j] = temp;

Solution

1. Step 1: Use a temporary variable for swapping

   To swap two values, store one in a temporary variable to avoid overwriting.

2. Step 2: Assign values correctly

   Assign arr[i] to temp, then arr[j] to arr[i], then temp to arr[j].

3. Final Answer:

   int temp = arr[i]; arr[i] = arr[j]; arr[j] = temp; -> Option C

4. Quick Check:

   Swap uses temp variable [OK]

Hint: Always use a temp variable to swap two elements safely [OK]

Common Mistakes:

• Overwriting one element before saving it
• Using uninitialized temp variable
• Swapping without temp causes data loss

3. What is the output of the following C code?

int arr[] = {1, 2, 3, 4, 5};
int n = 5;
for(int i = 0; i < n/2; i++) {
  int temp = arr[i];
  arr[i] = arr[n - i - 1];
  arr[n - i - 1] = temp;
}
for(int i = 0; i < n; i++) {
  printf("%d ", arr[i]);
}

medium

A. 2 1 4 3 5

B. 5 4 3 2 1

C. 1 2 3 4 5

D. 5 3 2 4 1

Solution

1. Step 1: Understand the loop swaps

   The loop swaps elements from start and end moving towards the center: arr[0] with arr[4], arr[1] with arr[3].

2. Step 2: Trace swaps

   Swap 1 and 5 -> arr = {5, 2, 3, 4, 1}; Swap 2 and 4 -> arr = {5, 4, 3, 2, 1}; middle element 3 stays same.

3. Final Answer:

   5 4 3 2 1 -> Option B

4. Quick Check:

   Reversed array = 5 4 3 2 1 [OK]

Hint: Swapping pairs from ends reverses array [OK]

Common Mistakes:

• Not swapping all pairs correctly
• Loop running too many or too few times
• Confusing indices in swap

4. Identify the error in this C code snippet intended to reverse an array:

int arr[] = {10, 20, 30, 40};
int n = 4;
for(int i = 0; i <= n/2; i++) {
  int temp = arr[i];
  arr[i] = arr[n - i - 1];
  arr[n - i - 1] = temp;
}

medium

A. Temporary variable temp is not declared

B. The swap logic is incorrect

C. Array size n is wrong

D. The loop condition should be i < n/2 instead of i <= n/2

Solution

1. Step 1: Analyze loop condition

   The loop runs from i = 0 to i <= n/2, which means it runs one extra iteration when i = n/2.

2. Step 2: Understand effect of extra iteration

   For even n=4, n/2=2, so i=2 is invalid index for swapping (out of bounds or swaps middle element twice).

3. Final Answer:

   The loop condition should be i < n/2 instead of i <= n/2 -> Option D

4. Quick Check:

   Loop runs too far, fix condition [OK]

Hint: Use i < n/2 to avoid extra swap [OK]

Common Mistakes:

• Using <= causes out-of-bound swaps
• Confusing array length with index
• Ignoring zero-based indexing

5. Given an array arr of size n, which approach is best to reverse the array in-place with minimum extra memory and why?

hard

A. Use two pointers, one at start and one at end, swap and move towards center

B. Create a new array and copy elements from end to start

C. Use recursion to swap elements from ends

D. Sort the array in descending order

Solution

1. Step 1: Compare memory usage of methods

   Creating a new array uses extra memory equal to array size; recursion uses call stack memory.

2. Step 2: Evaluate two-pointer method

   Two pointers swap elements in-place without extra memory, making it efficient and simple.

3. Final Answer:

   Use two pointers, one at start and one at end, swap and move towards center -> Option A

4. Quick Check:

   Two-pointer swap = minimal memory [OK]

Hint: Two-pointer swap reverses in-place with no extra memory [OK]

Common Mistakes:

• Using extra arrays wastes memory
• Recursion adds unnecessary stack overhead
• Sorting does not reverse original order