using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var alternating  = new double[]{12,10,11,9,23,17};

        var shouldBeTrue = IsAlternating(alternating);

        Console.WriteLine(shouldBeTrue); //true

        var notAlternating  = new double[]{13,12,10,11,9,23,17};

        var shouldBeFalse = IsAlternating(notAlternating);

        Console.WriteLine(shouldBeFalse);//false

    }

    public static bool IsAlternating(double[] data)

    {

        var d = GetDerivative(data);

​

        var signs = d.Select(val => Math.Sign(val));

​

        bool isAlternating =

       signs.Zip(signs.Skip(1), (a, b) => a != b).All(isAlt => isAlt);

        return isAlternating;

    }

​

    private static IEnumerable<double> GetDerivative(double[] data)

    {

        var d = data.Zip(data.Skip(1), (a, b) => b - a);

        return d;

    }

}