// input array is assumed to be sorted
public int BinarySearch(int[] arr, int x)
{
    if (arr.Length == 0)
        return -1;
 
    int mid = arr.Length / 2;
 
    if (arr[mid] == x)
        return mid;
 
    if (x < arr[mid])
        return BinarySearch(GetSubArray(arr, 0, mid - 1), x);
    else
    {
        int _indexFound = BinarySearch(GetSubArray(arr, mid + 1, arr.Length - 1), x);
        if (_indexFound == -1)
            return -1;
        else
            return mid + 1 + BinarySearch(GetSubArray(arr, mid + 1, arr.Length - 1), x);
    }
}
 
 
public int[] GetSubArray(int[] arr, int start, int end)
{
    List<int> _result = new List<int>();
    for (int i = start; i <= end; i++)
    {
        _result.Add(arr[i]);
    }
    return _result.ToArray();
}