using System;

using System.Collections.Generic;

​

public class Program

{

    public void Main()

    {

        var list = new List<int> {1,3,100,5,7,7,7,-1,7,17,8,9,0,9,12,18,18};

        var number = 7;

        list.Sort();

        Console.WriteLine((BinarySearch(list, number, list.Count, false) - BinarySearch(list, number, list.Count, true) + 1) + " time(s)");

    }

​

    int BinarySearch(List<int> list, int item, int len, bool isLower)

    {

        --len;

        int start = 0, mid, index = -1;

        while (start <= len)

        {

            mid = (start + len) / 2;

            if (item == list[mid])

            {

                index = mid;

                if (isLower)

                    len = mid - 1;

                else

                    start = mid + 1;

            }

            else if (item < list[mid])

            {

                len = mid - 1;

            }

            else if (item > list[mid])

            {

                start = mid + 1;

            }

        }

        return index;

    }

}