Day 003 - Reverse an Array (Arrays).md

🔁 Reverse an Array 🔁

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🧩 Problem Statement

You are given an array of integers arr[].
Your task is to reverse the given array in place.
That means no extra array — just flip it inside out, right where it is! 🌀

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🧪 Examples

✅ Example 1:

Input:

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

Output:

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

Explanation:

• Original array: 1 4 3 2 6 5
• After reversing: 5 6 2 3 4 1

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✅ Example 2:

Input:

arr = [4, 5, 2]

Output:

[2, 5, 4]

Explanation:

• Start with two pointers: i = 0, j = 2
• Swap arr[0] and arr[2] → [2, 5, 4]
• Now i = 1, j = 1 (both meet at the center), so stop.
• Final result is the reversed array: [2, 5, 4]

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✅ Example 3:

Input:

arr = [1]

Output:

[1]

Explanation:
Single element remains unchanged.

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🧠 Approach

💡 Idea:
We simply swap elements from both ends moving toward the center.

⚙️ Steps:

1. Start with two pointers — one at the beginning (i), and one at the end (n - i - 1).
2. Keep swapping these elements until you reach the middle.
3. Done! 🎉

This is the classic in-place reverse strategy — minimal and elegant!

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⏱️ Time & Space Complexity

Complexity	Value	Reason
🕒 Time	O(n/2) → O(n)	Half swaps, but still linear in nature
📦 Space	O(1)	In-place, no extra memory used

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🎯 Constraints

• 1 ≤ arr.length ≤ 10⁵
• 0 ≤ arr[i] ≤ 10⁵

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💻 Code

class Solution {
    public void reverseArray(int arr[]) {
        int n = arr.length;
        for(int i = 0; i < n / 2; i++){
            int temp = arr[n - i - 1];
            arr[n - i - 1] = arr[i];
            arr[i] = temp;
        }
    }
}

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Made with ❤️ by Milan Haria