QuickSort - C#

Efficiency: O(nlog n)

        public static int[] QuickSort(int[] arr)
        {
            if (arr.Length <= 1)
                return arr;

            int pivot = arr.Length - 1;

            int[] less = GetLessThanEqualToPivot(arr, pivot);
            int[] greater = GetGreaterThanPivot(arr, pivot);

            return Concatenate(QuickSort(less), arr[pivot], QuickSort(greater));
        }

        public static int[] Concatenate(int[] less, int pivotElement, int[] greater)
        {
            List
 
   _result = new List
  
   ();
            _result.AddRange(less);
            _result.Add(pivotElement);
            _result.AddRange(greater);
            return _result.ToArray();
        }

        public static int[] GetLessThanEqualToPivot(int[] arr, int pivot)
        {
            List
   
     _result = new List
    
     ();

            for (int i = 0; i < arr.Length - 1; i++)
            {
                if (arr[i] <= arr[pivot])
                {
                    _result.Add(arr[i]);
                }
            }

            return _result.ToArray();
        }

        public static int[] GetGreaterThanPivot(int[] arr, int pivot)
        {
            List
     
       _result = new List
      
       ();
            for (int i = 0; i < arr.Length - 1; i++)
            {
                if (arr[i] > arr[pivot])
                {
                    _result.Add(arr[i]);
                }
            }
            return _result.ToArray();
        }