Array Reversal Techniques in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of reversing an array using slicing

What is the output of the following Python code that reverses an array using slicing?

DSA Python

arr = [10, 20, 30, 40, 50]
reversed_arr = arr[::-1]
print(reversed_arr)

A [10, 20, 30, 40, 50]

B [50, 40, 30, 20]

C [50, 40, 30, 20, 10]

D [10, 20, 30, 40]

💡 Hint

Remember that slicing with [::-1] reverses the list.

Using arr[::-1] creates a new list with elements in reverse order. So the output is the original list reversed.

❓ Predict Output

intermediate

Output after reversing array in-place with two pointers

What is the output of this code that reverses an array in-place using two pointers?

DSA Python

arr = [1, 2, 3, 4, 5]
left, right = 0, len(arr) - 1
while left < right:
    arr[left], arr[right] = arr[right], arr[left]
    left += 1
    right -= 1
print(arr)

A [5, 2, 3, 4, 1]

B [1, 2, 3, 4, 5]

C [1, 5, 3, 2, 4]

D [5, 4, 3, 2, 1]

💡 Hint

Swapping elements from both ends moves the array towards reversed order.

The two-pointer approach swaps elements from the start and end moving inward, reversing the array in-place.

🔧 Debug

advanced

Identify the error in this array reversal function

What error does this code produce when trying to reverse an array?

DSA Python

def reverse_array(arr):
    for i in range(len(arr) // 2):
        arr[i], arr[len(arr) - i - 1] = arr[len(arr) - i - 1], arr[i]

arr = [1, 2, 3, 4]
reverse_array(arr)
print(arr)

A No error, prints [4, 3, 2, 1]

B IndexError

C TypeError

D SyntaxError

💡 Hint

Check the index used for swapping elements.

The code uses len(arr) - i which is out of range because list indices go from 0 to len(arr)-1.

🧠 Conceptual

advanced

Number of swaps needed to reverse an array of length n

How many element swaps are needed to reverse an array of length n using the two-pointer method?

A n // 2

B n // 3

C n - 1

D n

💡 Hint

Each swap fixes two elements at once.

Each swap exchanges two elements, so only half the array length swaps are needed.

🚀 Application

expert

Output after reversing a nested array with slicing

What is the output of this code that reverses a nested array using slicing?

DSA Python

arr = [[1, 2], [3, 4], [5, 6]]
reversed_arr = arr[::-1]
print(reversed_arr)

A [[5, 6], [3, 4], [1, 2]]

B [[1, 2], [3, 4], [5, 6]]

C [[6, 5], [4, 3], [2, 1]]

D [[5, 6], [4, 3], [1, 2]]

💡 Hint

Slicing reverses the outer list but does not reverse inner lists.

The slicing reverses the order of the inner lists but does not change their contents.