using System;

using System.Diagnostics;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        Stopwatch timer = new Stopwatch();

        Random randomizer = new Random();

        int arraySize = randomizer.Next(100000, 1000000);

        List<int> randomIntegers = new List<int>(arraySize);

        for (int index = 0; index < arraySize; ++index)

        {

            randomIntegers.Add(randomizer.Next(int.MinValue, int.MaxValue));

        }

​

        Console.WriteLine("Total: {0}", arraySize);

        int[] numbers = randomIntegers.ToArray();

        timer.Restart();

        SecondSeven(numbers); // O(n)

        timer.Stop();

        Console.WriteLine("Elapsed (ms): {0}", timer.ElapsedMilliseconds);

        timer.Restart();

        LinqSeven(numbers);

        timer.Stop();

        Console.WriteLine("Elapsed (ms): {0}", timer.ElapsedMilliseconds);

    }

​

    public static void SecondSeven(int[] numbers)

    {

        int ? first = null, second = null;

        foreach (var number in numbers)

        {

            if (number % 7 == 0)

            {

                if (first == null)

                {

                    first = number;

                }

                else if (number > first.GetValueOrDefault())

                {

                    second = first;

                    first = number;

                }

                else if (second == null)

                {

                    second = number;

                }

                else if (number > second.GetValueOrDefault())

                {

                    second = number;

                }

            }

        }

​

        Console.WriteLine("First Seven: {0} Second Seven: {1}", first, second);

    }

​

    public static void LinqSeven(int[] numbers)

    {

        var sevens = numbers.Where(x => x % 7 == 0).ToList();

        sevens.Sort();

        int count = sevens.Count();

        if (count >= 2)

        {

            Console.WriteLine("Second Seven: {0}", sevens[count - 2]);

        }

        else

        {

            Console.WriteLine("No second seven");

        }

​

        Console.WriteLine("Count: {0}", count);

    }

}