Array Reversal Techniques in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of reversing an array using a for loop

What is the output of the following C# code that reverses an array using a for loop?

DSA C#

int[] arr = {1, 2, 3, 4, 5};
int n = arr.Length;
for (int i = 0; i < n / 2; i++) {
    int temp = arr[i];
    arr[i] = arr[n - i - 1];
    arr[n - i - 1] = temp;
}
foreach (int num in arr) {
    Console.Write(num + " ");
}

A 5 4 3 2 1

B 1 2 3 4 5

C 5 2 3 4 1

D 1 4 3 2 5

💡 Hint

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

The for loop swaps elements from the start and end of the array until the middle is reached, effectively reversing the array.

❓ Predict Output

intermediate

Output of reversing an array using Array.Reverse method

What is the output of the following C# code that uses Array.Reverse to reverse an array?

DSA C#

int[] arr = {10, 20, 30, 40};
Array.Reverse(arr);
foreach (int num in arr) {
    Console.Write(num + " ");
}

A 30 20 10 40

B 10 20 30 40

C 40 20 30 10

D 40 30 20 10

💡 Hint

Array.Reverse reverses the entire array in place.

Array.Reverse reverses the elements of the array in place, so the output is the reversed array.

🔧 Debug

advanced

Identify the error in this array reversal code

What error will this C# code produce when trying to reverse an array?

DSA C#

int[] arr = {1, 2, 3, 4};
int n = arr.Length;
for (int i = 0; i <= n / 2; i++) {
    int temp = arr[i];
    arr[i] = arr[n - i - 1];
    arr[n - i - 1] = temp;
}
foreach (int num in arr) {
    Console.Write(num + " ");
}

A No error, outputs: 4 3 3 4

B No error, outputs: 4 3 2 1

C IndexOutOfRangeException

D Compile-time error

💡 Hint

Check the loop condition and array indexing carefully.

The loop runs one step too far causing an index out of range when accessing arr[n - i - 1] at i = n/2.

🧠 Conceptual

advanced

Time complexity of array reversal

What is the time complexity of reversing an array of size n using the standard two-pointer swap method?

A O(n)

B O(n^2)

C O(log n)

D O(1)

💡 Hint

Consider how many swaps are done relative to the array size.

Each element is swapped once, so the number of operations grows linearly with n.

🚀 Application

expert

Output after reversing a subarray

Given the array arr = {5, 10, 15, 20, 25, 30} and the code below that reverses a subarray from index 1 to 4, what is the final array?

DSA C#

int[] arr = {5, 10, 15, 20, 25, 30};
int start = 1, end = 4;
while (start < end) {
    int temp = arr[start];
    arr[start] = arr[end];
    arr[end] = temp;
    start++;
    end--;
}
foreach (int num in arr) {
    Console.Write(num + " ");
}

A 5 20 15 10 25 30

B 5 25 20 15 10 30

C 30 25 20 15 10 5

D 5 10 15 20 25 30

💡 Hint

Only the elements between index 1 and 4 are reversed.

The subarray {10, 15, 20, 25} is reversed to {25, 20, 15, 10}, rest remain same.

Practice

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

easy

A. Removing duplicate elements from the array

B. Sorting the array in ascending order

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

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 in a sequence.

2. Step 2: Define reversal

   Reversing means swapping elements so the last element moves to the front and the first moves to the end, flipping the order.

3. Final Answer:

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

4. Quick Check:

   Reversal = Flip 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 is the correct syntax to reverse an array named arr in C# using built-in methods?

easy

A. Array.Reverse(arr);

B. arr.Reverse();

C. reverse(arr);

D. arr.reverse();

Solution

1. Step 1: Identify C# array reversal method

   C# provides a static method Array.Reverse() to reverse arrays in place.

2. Step 2: Check syntax correctness

   The correct syntax calls Array.Reverse(arr); with capital 'A' and 'R' and no return value.

3. Final Answer:

   Array.Reverse(arr); -> Option A

4. Quick Check:

   Use Array.Reverse(arr) in C# [OK]

Hint: Use Array.Reverse(arr) to reverse arrays in C# [OK]

Common Mistakes:

• Using lowercase method names
• Calling reverse as instance method on array
• Using non-existent reverse functions

3. What will be the output of the following C# code?

int[] arr = {1, 2, 3, 4, 5};
Array.Reverse(arr);
Console.WriteLine(string.Join(", ", arr));

medium

A. 1, 2, 3, 4, 5

B. 5, 2, 3, 4, 1

C. Syntax Error

D. 5, 4, 3, 2, 1

Solution

1. Step 1: Understand Array.Reverse effect

   The method reverses the array elements in place, so the order flips.

2. Step 2: Trace the output

   Original array: 1, 2, 3, 4, 5 becomes 5, 4, 3, 2, 1 after reversal.

3. Final Answer:

   5, 4, 3, 2, 1 -> Option D

4. Quick Check:

   Reversed array prints 5 to 1 [OK]

Hint: Reversal flips array order exactly [OK]

Common Mistakes:

• Assuming reversal sorts the array
• Expecting original order after reversal
• Confusing output format

4. Identify the error in the following C# code that attempts to reverse an array:

int[] arr = {10, 20, 30};
arr.Reverse();
Console.WriteLine(string.Join(", ", arr));

medium

A. arr.Reverse() does not reverse the array in place

B. Array must be converted to list before reversing

C. Console.WriteLine syntax is incorrect

D. No error, code works fine

Solution

1. Step 1: Check arr.Reverse() method

   In C#, arr.Reverse() returns a reversed sequence but does not change the original array.

2. Step 2: Understand effect on original array

   The original array remains unchanged, so printing it shows original order, not reversed.

3. Final Answer:

   arr.Reverse() does not reverse the array in place -> Option A

4. Quick Check:

   arr.Reverse() returns reversed IEnumerable, no in-place change [OK]

Hint: Array.Reverse(arr) reverses in place; arr.Reverse() does not [OK]

Common Mistakes:

• Assuming arr.Reverse() changes original array
• Confusing LINQ Reverse with Array.Reverse
• Ignoring return value of arr.Reverse()

5. Given an integer array arr with an odd number of elements, which approach correctly reverses the array in place without using built-in methods?

hard

A. Create a new array and copy elements from end to start, then assign back to original array

B. Use two pointers, one at start and one at end, swap elements and move pointers towards center until they meet

C. Sort the array in descending order to reverse it

D. Remove the middle element and reverse the rest of the array

Solution

1. Step 1: Understand in-place reversal with two pointers

   Start with one pointer at the first element and another at the last element, swap their values.

2. Step 2: Move pointers towards center and swap

   Increment the start pointer and decrement the end pointer, continue swapping until pointers meet or cross.

3. Final Answer:

   Use two pointers, one at start and one at end, swap elements and move pointers towards center until they meet -> Option B

4. Quick Check:

   Two-pointer swap reverses array in place [OK]

Hint: Swap start and end elements moving inward [OK]

Common Mistakes:

• Using sorting instead of reversal
• Creating extra arrays unnecessarily
• Removing elements instead of swapping