Float numbers and operators - try to put n - 1 operators to get maximum value, n is the float number's length

FloatNumberAndOperators.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace FindMaximumNumber
{    
    /*
    Keyword:

    a list of float number
    four operator: +, -, *, / algebra

    insert operator between each consecutive pair of numbers    

    Ask:
    Find maximimum value -> greedy algorithm    

    Constraints:
    equal precedence order - 1 + 2 * 3 = 1 + 6 = 7 - No
                             1 + 2 * 3 = 3 * 3 = 9 - Yes
    evaluation order is from left to right 
    -> scan the float number from left to right 
    1, 12, -3, maximum number is 1 - 12 * (-3) = 33
            1     
       /  /  \    \
     -11 13  1/12 12  -> Maximum and Minimum
    */
    class Program
    {
        static void Main(string[] args)
        {
            RunTestcase(); 
        }

        public static void RunTestcase1()
        {
            var result = GetMaxNumber(new double[] { 1, 12});
        }

        public static void RunTestcase()
        {
            var input = new double[] { 1, 12, -3 };            

            var result = GetMaxNumber(input);
            Debug.Assert(result[0] == 33); 
        }

        public static double[] GetMaxNumber(double[] numbers)
        {
            if (numbers == null || numbers.Length == 0)
            {
                return new double[] { }; 
            }

            return GetMaxNumberHelper(numbers, 1, new double[] { numbers[0], numbers[0] });
        }

        /// <summary>
        /// code review 1/29/2018
        /// </summary>
        /// <param name="numbers"></param>
        /// <param name="startIndex"></param>
        /// <param name="original"></param>
        /// <returns></returns>
        public static double[] GetMaxNumberHelper(double[] numbers, int startIndex, double[] original)
        {
            if (startIndex >= numbers.Length)
            {
                return original; 
            }

            var length = numbers.Length;

            var visit = numbers[startIndex];
            
            var fourOperations = getFourOperations(original, visit);

            var current = new double[] { fourOperations.Max(), fourOperations.Min() };

            return GetMaxNumberHelper(numbers, startIndex + 1, current);
        }

        /// <summary>
        /// code review 1/29/2018
        /// </summary>
        /// <param name="maxMin"></param>
        /// <param name="number2"></param>
        /// <returns></returns>
        private static double[] getFourOperations(double[] maxMin, double number2)
        {
            var addition    = maxMin[0] + number2;
            var addition2   = maxMin[1] + number2; 

            var subtraction  = maxMin[0] - number2;
            var subtraction2 = maxMin[1] - number2;  

            var multiplication  = maxMin[0] * number2;
            var multiplication2 = maxMin[1] * number2; 

            var division  = maxMin[0] / number2;
            var division2 = maxMin[1] / number2;

            return new double[] { addition, addition2, subtraction, subtraction2, multiplication, multiplication2, division, division2 };            
        }       
    }
}