Reverse an Array-java

My Thought Process (Java DSA)

When I first looked at this problem on GeeksforGeeks, it seemed very simple. But instead of jumping directly into code, I tried to understand what is actually happening when we "reverse" an array.

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Problem Statement

We are given an array of integers arr[].
The task is to reverse the array in-place, meaning we should not use any extra space.

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Initial Thinking

If I take an example:

arr = [1, 4, 3, 2, 6, 5]

The reversed array should be:

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

At first, I thought about creating a new array and filling it from the end. But then I noticed the constraint: modify the array in-place.

So using extra space is not a good approach here.

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Key Observation

Reversing an array means:

• First element goes to last
• Second element goes to second last
• And so on...

So instead of moving everything, I can just swap elements from both ends.

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Approach (Two Pointer Technique)

I used two pointers:

• left starting from index 0
• right starting from index n - 1

Then I keep swapping:

• arr[left] with arr[right]
• Move left++
• Move right--

Stop when left >= right

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It should be Run like

arr = [1, 4, 3, 2, 6, 5]

Step 1:
swap(1, 5) → [5, 4, 3, 2, 6, 1]

Step 2:
swap(4, 6) → [5, 6, 3, 2, 4, 1]

Step 3:
swap(3, 2) → [5, 6, 2, 3, 4, 1]

Done

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Code in (Java)

class Solution {
    public void reverseArray(int arr[]) {
        int left = 0;
        int right = arr.length - 1;

        while (left < right) {
            int temp = arr[left];
            arr[left] = arr[right];
            arr[right] = temp;

            left++;
            right--;
        }
    }
}

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How This Works

• We are not shifting elements
• We are just swapping pairs
• Each swap puts two elements in correct position
• We only need to go till the middle of the array

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Complexity Analysis

• Time Complexity: O(n)
• Space Complexity: O(1)

This is optimal because:

• We traverse the array only once
• No extra memory is used

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Final Thought

This problem is a basic example of the two-pointer technique.
Understanding this helps in many other problems like:

• Palindrome checking
• Two sum (sorted arrays)
• String reversal

Instead of thinking complex, sometimes the best solution is just swapping from both ends.

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