How to reverse an array without using extra space in O(n) time

Using technique of swapping two numbers without using extra space can work here.

    static void Main(string[] args)
    {
        int[] arr = new int[] { 1, 2, 3, 4, 5, 6, 7, 8 };

        for (int i = 0; i < arr.Length / 2; i++)
        {
            // In first iteration
            // arr[0] = 1+8 = 9
            arr[i] += arr[arr.Length - i - 1];
            // arr[7] = 9-8 = 1
            arr[arr.Length - i - 1] = arr[i] - arr[arr.Length - i - 1];
            // arr[0] = 9-1 = 8
            arr[i] = arr[i] - arr[arr.Length - i - 1];
        }

        foreach (int element in arr)
            Console.WriteLine(element);
    }

Output:

8

7

6

5

4

3

2

1