353. Design Snake Game

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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:

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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)

解法1： Deque + HashMap

设计题在于选用合适的数据结构来完成要求的feature。
用一个hashmap存每一个snake的位置是便于查询是否头咬到了身体
用deque来表示一个snake，方便从头从尾巴更新。
每次移动的时候先要把tail移除，然后判断是否溢出，是否咬到了身体。
然后再判断是否吃到了果实，如果是那么再把tail给放回去。

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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]. */
    Deque<Integer> snake;
    Set<Integer> snakeBody;
    int width;
    int height;
    int[][] food;
    int score;
    int foodIndex;
    public SnakeGame(int width, int height, int[][] food) {
        this.snake = new LinkedList<Integer>();
        this.snakeBody = new HashSet<>();
        this.width = width;
        this.height = height;
        this.food = food;
        this.score = 0;
        this.foodIndex = 0;
        snake.offerFirst(0);
        snakeBody.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 score;
        }
        int head = snake.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;
        }
        // remove tail from snake
        int tail = snake.peekLast();
        snake.removeLast();
        snakeBody.remove(tail);
        // check if snake is out of boundary
        if (row < 0 || row >= height || col < 0 || col >= width) {
            score = -1;
            return score;
        }
        int newHead = row * width + col;
        if (snakeBody.contains(newHead)) {
            score = -1;
            return score;
        }
        // check if the tail needs to be added back
        if (foodIndex < food.length) {
            int[] currentFood = food[foodIndex];
            if (row == currentFood[0] && col == currentFood[1]) {
                snake.offerLast(tail);
                snakeBody.add(tail);
                foodIndex++;
                score++;
            }            
        }
        // add new head into snake
        snake.offerFirst(newHead);
        snakeBody.add(newHead);
        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);
 */