Solution of codingame contest ("Telephone numbers")

This is a solution of the following task
http://www.codingame.com/ide/?target=clogin&s=1&id=17493999f3957445a56a9161124e4ab5b88aa6#!test:192854:true:%2523!list

By joining the iDroid smartphone development team, you have been given the responsibility of developing the contact manager. Obviously, what you were not told is that there are strong technical constraints for iDroid: the system doesn’t have much memory and the processor is as fast as a Cyrix from the 90s...

In the specifications, there are two points in particular that catch your attention:

1. Intelligent Assistance for entering numbers
The numbers corresponding to the first digits entered will be displayed to the user almost instantly.

2. Number storage optimization
First digits which are common to the numbers should not be duplicated in the memory.

Fortunately, the specifications also have this little chart to guide you in the implementation:

Fig 1. Structure of data to stock phone numbers on iDroid

Your task is to write a program that displays the number of items (which are numbers) required to store a list of telephone numbers with the structure presented above.

INPUT:

Line 1: The number N of telephone numbers.

N Following lines: Each line contains a phone number, with a maximum length L. Telephone numbers consist of only the digits 0 to 9 included, without any spaces.

OUTPUT:

The number of elements (referencing a number) stored in the structure.

CONSTRAINTS:

0 ≤ N ≤ 10000
2 ≤ L ≤ 20

EXAMPLES:

Input

1

0467123456

Output

10

Input

2
0123456789
1123456789

Output

20

Input

2
0123456789
0123

Output

10

Input

5
0412578440
0412199803
0468892011
112
15

Output

28

---

Available RAM : 512MB

Timeout: 1 seconds

• The program has to read inputs from standard input
• The program has to write the solution to standard output
• The program must run in the test environment

Download the files provided in the test script:

A telephone number: in1.txt out1.txt

Numbers with a different base: in2.txt out2.txt

Number included in another: in3.txt out3.txt

Numbers with a common part: in4.txt out4.txt

Long list of numbers: in5.txt out5.txt

Proposed Solution in C#

Code:

static class PhoneNumbers
    {
        

        public static void Run()
        {
            List<string> numbers = new List<string>();
            string line = Console.ReadLine();
            //int item = 0;
            int n = Convert.ToInt32(line);
            if (n > 0)
            {
                for (int i = 0; i < n; i++)
                {
                    line = Console.ReadLine();
                    numbers.Add(line);
                }
            }

            BuildTree(numbers.Distinct().ToList());

            Console.WriteLine(counter);
        }

        static int counter = 0;

        static void BuildTree(List<string> list)
        {
            if (list.Count == 1)
            {
                counter += list[0].Length;
                return;
            }


            for (int i = 0; i <= 9; i++)
            {
                var started = (from item in list where item.StartsWith(i.ToString()) select item).Distinct().ToList();
                if (started.Count > 0)
                {
                    counter++;
                    var newList = (from item in started where item.Length > 1 select item.Substring(1)).ToList();
                    BuildTree(newList);
                }
            }
        }
    }