Reverse an Array

Introduction

Arrays are one of the most basic and widely used data structures in programming.
A common operation performed on arrays is reversing the elements, which means rearranging the elements in opposite order.

This problem helps us understand indexing, swapping, and basic logic building.

Problem Statement

Given an array, reverse its elements such that:

• The first element becomes the last
• The second element becomes the second-last
• And so on

Examples

Example 1:
Input: [1, 4, 3, 2, 6, 5]
Output: [5, 6, 2, 3, 4, 1]

Example 2:
Input: [4, 5, 1, 2]
Output: [2, 1, 5, 4]

Intuition

To reverse an array, we need to swap elements from both ends:

• First element ↔ Last element
• Second element ↔ Second-last element

We continue this until we reach the middle of the array.

Approach

1. Initialize two pointers: start = 0 end = n - 1
2. Swap elements at start and end
3. Move: start++ end--
4. Repeat until start < end

Code (Python)

def reverse_array(arr):
    start = 0
    end = len(arr) - 1

    while start < end:
        # Swap elements
        arr[start], arr[end] = arr[end], arr[start]

        start += 1
        end -= 1

    return arr

Step-by-Step Explanation

For array:
[1, 4, 3, 2, 6, 5]

Swap 1 and 5 → [5, 4, 3, 2, 6, 1]
Swap 4 and 6 → [5, 6, 3, 2, 4, 1]
Swap 3 and 2 → [5, 6, 2, 3, 4, 1]

Now start >= end, so we stop.

Complexity Analysis

• Time Complexity: O(n)
• Space Complexity: O(1) (in-place reversal)

Conclusion

Reversing an array is a simple yet important problem that builds a strong foundation in problem-solving. It introduces concepts like two-pointer technique and in-place operations, which are widely used in coding interviews.