Design Snake Game

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)

Tips:

看起来难实际上比较简单，重点是用Deque，可以两头加减蛇。加入HashSet可以快速读取蛇有没有咬到自己。先计算移动后新蛇头的位置，判断是否出局，然后看是否吃了食物，吃了则蛇头入队，不减蛇尾，没吃则是普通移动，蛇头入队，减去老蛇尾。

Code：

public class SnakeGame {

    /** 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]. */
        HashSet<Integer> set;
        Deque<Integer> body;
        int score;
        int[][] food;
        int foodIndex;
        int width;
        int height;
    public SnakeGame(int width, int height, int[][] food) {
        this.width = width;
        this.height = height;
        this.food = food;
        set = new HashSet<Integer>();
        set.add(0);
        body = new LinkedList<Integer>();
        body.addLast(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;
        }
        //calculate head's position
        int rowHead = body.peekFirst() / width;
        int colHead = body.peekFirst() % width;
        // move
        switch (direction) {
            case "U" : rowHead--;
                       break;
            case "D" : rowHead++;
                       break;
            case "L" : colHead--;
                       break;
            case "R" : colHead++;
                       break;
        }
        set.remove(body.peekLast()); // since the snake moved, it is no longer in the old tail's position.
        int headPos = rowHead * width + colHead; // recaluate new head position
        //Game Over
        if (rowHead < 0 || rowHead == height || colHead < 0 || colHead == width || set.contains(headPos)) {
            return score = -1;
        }
        //add head
        set.add(headPos);
        body.addFirst(headPos);
        // eating food, keep tail, add head
        if (foodIndex < food.length && rowHead == food[foodIndex][0] && colHead == food[foodIndex][1]) {
            set.add(body.peekLast()); // old tail does not change, so add it back to set
            foodIndex++;
            return ++score;
        }
        // if didnt eat food, remove old tail,just normal move
        body.removeLast();
        return score;
    }
}

/**
 * Your SnakeGame object will be instantiated and called as such:
 * SnakeGame obj = new SnakeGame(width, height, food);
 * int param_1 = obj.move(direction);
 */