using System;

using System.Text;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Exercise 1 - For the purpose of this exercise we ignore types based on angles, which require complex sin and cos computations.");

        Console.WriteLine("Test Dimensions (3,3,3): " + GetTriangleType(3, 3, 3));

        Console.WriteLine("Test Dimensions (3,2,3): " + GetTriangleType(3, 2, 3));

        Console.WriteLine("Test Dimensions (3,2,1): " + GetTriangleType(3, 2, 1));

        Console.WriteLine("=====");

        Console.WriteLine("Exercise 2");

        var str = "Test String 1";

        Console.WriteLine(str + " : " + ReverseSentence(str));

        str = "! Test Str!ng5 2";

        Console.WriteLine(str + " : " + ReverseSentence(str));

        Console.WriteLine("=====");

        Console.WriteLine("Exercise 3 - By definition, either index-based structures or doubly linked lists would allow constant instead of linear retrieval.");

        Program prog = new Program();

        var list = new CustomList();

        list.Push(1);

        list.Push(2);

        list.Push(3);

        list.Push(4);

        list.Push(5);

        list.Push(6);

        list.Push(7);

        Console.WriteLine(list.GetNthElementFromBack(5));

        list.Push(8);

        Console.WriteLine(list.GetNthElementFromBack(5));

    }

​

    public enum TriangleType

    {

        Equilateral,

        Iscosceles,

        Scalene

    }

​

    public static TriangleType GetTriangleType(int a, int b, int c)

    {

        if (a < 1 || b < 1 || c < 1)

            throw new InvalidOperationException("Invalid Parameters Given");

        int[] arr = {a, b, c};

        Array.Sort(arr);

        if (arr[0] == arr[2])

        {

            return TriangleType.Equilateral;

        }

​

        if (arr[0] == arr[1] || arr[1] == arr[2])

        {

            return TriangleType.Iscosceles;

        }

        else

            return TriangleType.Scalene;

    }

​

    public static string ReverseSentence(string input)

    {

        var arr = input.ToCharArray();

        Array.Reverse(arr);

        return new string (arr);

    }

​

    public class CustomNode

    {

        public CustomNode(int val)

        {

            this.Value = val;

        }

​

        public int Value

        {

            get;

            set;

        }

​

        public CustomNode Next

        {

            get;

            set;

        }

    }

​

    public class CustomList

    {

        public CustomList()

        {

            this.Count = 0;

        }

​

        public CustomNode First;

        public CustomNode Last;

        public int Count

        {

            get;

            set;

        }

​

        public void Push(int value)

        {

            var node = new CustomNode(value);

            if (Count < 1)

            {

                this.First = node;

            }

            else

            {

                Last.Next = node;

            }

​

            this.Last = node;

            Count++;

        }

​

        public int GetNthElementFromBack(int n)

        {

            var itemIndex = Count - n;

            if (itemIndex < 0)

                throw new InvalidOperationException("Invalid Parameters");

            var counter = 1;

            var elem = this.First;

            while (counter < itemIndex)

            {

                elem = elem.Next;

                counter++;

            }

​

            return elem.Value;

        }

    }

}