353. Design Snake Game

Problem:

Design a Snake game that is played on a device with screen size = width x height. Play the game online if you are not familiar with the game.

The snake is initially positioned at the top left corner (0,0) with length = 1 unit.

You are given a list of food's positions in row-column order. When a snake eats the food, its length and the game's score both increase by 1.

Each food appears one by one on the screen. For example, the second food will not appear until the first food was eaten by the snake.

When a food does appear on the screen, it is guaranteed that it will not appear on a block occupied by the snake.

Example:

Given width = 3, height = 2, and food = [[1,2],[0,1]].

Snake snake = new Snake(width, height, food);

Initially the snake appears at position (0,0) and the food at (1,2).

|S| | |
| | |F|

snake.move("R"); -> Returns 0

| |S| |
| | |F|

snake.move("D"); -> Returns 0

| | | |
| |S|F|

snake.move("R"); -> Returns 1 (Snake eats the first food and right after that, the second food appears at (0,1) )

| |F| |
| |S|S|

snake.move("U"); -> Returns 1

| |F|S|
| | |S|

snake.move("L"); -> Returns 2 (Snake eats the second food)

| |S|S|
| | |S|

snake.move("U"); -> Returns -1 (Game over because snake collides with border)

Analysis:
The behavior of snake is like deque. It can add to head, and delete from tail. Use a hash set to determine whether the snake bites itself.  We need to remove the tail when we are checking bite, because the snake moves one step further. If it eats food, add the tail back to the snake.

Solution:

class SnakeGame {
    List<Integer> snakeList = new ArrayList<>();
    int score;
    int foodIndex = 0;
    int width, height;
    int[][] food;
    Deque<Integer> deque;
    Set<Integer> set;
    /** Initialize your data structure here.
        @param width - screen width
        @param height - screen height 
        @param food - A list of food positions
        E.g food = [[1,1], [1,0]] means the first food is positioned at [1,1], the second is at [1,0]. */
    public SnakeGame(int width, int height, int[][] food) {
        deque = new LinkedList<>();
        set = new HashSet<>();
        this.width = width;
        this.height = height;
        this.food = food;
        deque.offerLast(0);
        set.add(0);
    }
    
    /** Moves the snake.
        @param direction - 'U' = Up, 'L' = Left, 'R' = Right, 'D' = Down 
        @return The game's score after the move. Return -1 if game over. 
        Game over when snake crosses the screen boundary or bites its body. */
    public int move(String direction) {
        if (score == -1) {
            return -1;
        }
        int head  = deque.peekFirst();
        int row = head/width;
        int col = head%width;
        switch(direction) {
            case "U" : row--;
                break;
            case "D" : row++;
                break;
            case "L" : col--;
                break;
            case "R" : col++;
                break;
        }
      
        head = row*width + col;
        set.remove(deque.peekLast());
        if (row < 0 || row >= height || col < 0 || col >= width || set.contains(head)) {
            score = -1;
            return score;
        }
           
        // move forward one step
        deque.offerFirst(head);
        set.add(head);
        // check food
        if (foodIndex < food.length && food[foodIndex][0] == row && food[foodIndex][1] == col) {
            foodIndex++;
            score++;
            set.add(deque.peekLast());
            return score;
        }
        // no food, delete tail
        deque.pollLast();
        return score;
    }
}