Reverse Array

Problem Link

Problem: Reverse an Array

Problem Statement

You are given an array of integers arr[]. Your task is to reverse the given array in place.

Note: Modify the array in-place without using extra space for another array.

Examples

Example 1
Input: arr = [1, 4, 3, 2, 6, 5]
Output: [5, 6, 2, 3, 4, 1]
Explanation: Elements are reversed by swapping from both ends toward the center.

Example 2
Input: arr = [4, 5, 2]
Output: [2, 5, 4]

Example 3
Input: arr = [1]
Output: [1]
Explanation: A single-element array remains the same after reversal.

Constraints

1 <= arr.length <= 10⁵
0 <= arr[i] <= 10⁵

Intuition

Reversing an array means swapping the first element with the last, the second with the second last, and so on. This two-pointer technique allows in-place reversal with minimal time and space.

Approach

Initialize two pointers:
- s = 0 (start)
- e = arr.length - 1 (end)
While s < e:
- Swap arr[s] and arr[e]
- Increment s and decrement e
Stop when pointers meet or cross.

Code (Java)

class Solution {
    public void reverseArray(int arr[]) {
        int n = arr.length;
        for (int s = 0, e = n - 1; s < e; s++, e--) {
            int temp = arr[s];
            arr[s] = arr[e];
            arr[e] = temp;
        }
    }
}

Note: Corrected loop condition to s < e to avoid unnecessary swaps when s == e.

Time and Space Complexity

Time Complexity: O(n / 2) ⇒ O(n)
Each element is visited once in a pairwise swap.
Space Complexity: O(1)
Swaps are done in-place without any additional data structures.

Dry Run

Input: `arr = [1, 4, 3, 2, 6, 5]`

s = 0, e = 5 → swap 1 and 5 → [5, 4, 3, 2, 6, 1]
s = 1, e = 4 → swap 4 and 6 → [5, 6, 3, 2, 4, 1]
s = 2, e = 3 → swap 3 and 2 → [5, 6, 2, 3, 4, 1]
Loop ends as s = 3, e = 2

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

Conclusion

This problem is a classic example of the two-pointer approach for in-place operations. The algorithm is simple, efficient, and optimal in both time and space for reversing arrays of any size.