using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var food = new int[,] { {1, 2}, {0, 1}};

        var snake = new Snake(3, 2, food);

        Console.WriteLine(snake.Move("R"));

        Console.WriteLine(snake.Move("D"));

        Console.WriteLine(snake.Move("R"));

        Console.WriteLine(snake.Move("U"));

        Console.WriteLine(snake.Move("L"));

        Console.WriteLine(snake.Move("L"));

        Console.WriteLine(snake.Move("D"));

        Console.WriteLine(snake.Move("R"));

    }

}

​

​

public class Snake

{

    private int _w;

    private int _h;

    private int _fIdx;

    private readonly int[,] _food;

    private LinkedList<Point> _snake;

    private HashSet<string> _hash;

    private int _score;

    public Snake(int width, int height, int[,] food)

    {

        _w = width;

        _h = height;

        _food = food;

        _fIdx = 0;

        _score = 0;

        _snake = new LinkedList<Point>();

        _hash = new HashSet<string>();

        AddFirst(0,0);

    }

    private string GetKey(int x, int y)

    {

        return string.Format("{0}.{1}", x, y);

    }

    private void AddFirst(int x, int y)

    {

        _snake.AddFirst(new Point(x, y));

        _hash.Add(GetKey(x,y));

    }

    private void RemoveLast()

    {

        var last = _snake.Last.Value;

        _snake.RemoveLast();

        _hash.Remove(GetKey(last.x, last.y));

    }

    private bool IsInvalid(int x, int y)

    {

        return x < 0 || x >= _w || y < 0 || y >= _h || _hash.Contains(GetKey(x,y));

    }

    private bool FindFood(int x, int y)

    {

        return _fIdx < _food.GetLength(0) && x == _food[_fIdx, 1] && y == _food[_fIdx, 0];

    }

    public int Move(string direction)

    {

        if (_score == -1)

            return _score;

        var cur = _snake.First.Value;

        var x = cur.x;

        var y = cur.y;

        switch(direction)

        {

            case "U":

                y--;

                break;

            case "D":

                y++;

                break;

            case "L":

                x--;

                break;

            case "R":

                x++;

                break;

        }

        if (IsInvalid(x, y))

            _score = -1;

        else if (FindFood(x,y))

        {

            AddFirst(x,y);

            _fIdx++;

            _score++;

        }

        else 

        {

            RemoveLast();

            AddFirst(x,y);

        }

        return _score;

    }

}

​

​

​

public class Point

{

    public int x;

    public int y;

    public Point(int x, int y)

    {

        this.x = x;

        this.y = y;

    }

}

​

​