Leetcode: 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)

HashSet + Queue:

吃东西的时候保留尾巴，不吃的时候删去尾巴

one case to notice蛇转弯的时候，要先删去尾巴，再把新的点加进去。如果这个顺序不对的话，本来不会撞上尾巴的结果会判断会撞上

public class SnakeGame {
    int m;
    int n;
    int[][] foods;
    Queue<Integer> queue;
    HashSet<Integer> set;
    int[] headPos;
    int foodSeq;


    /** 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) {
        this.m = height;
        this.n = width;
        this.foods = food;
        this.queue = new LinkedList<Integer>();
        this.set = new HashSet<Integer>();
        this.headPos = new int[]{0, 0};
        this.foodSeq = 0;
        queue.offer(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) {
        int[] dir;
        switch(direction) {
            case "U": dir = new int[]{-1, 0}; break;
            case "L": dir = new int[]{0, -1}; break;
            case "R": dir = new int[]{0, 1}; break;
            case "D": dir = new int[]{1, 0}; break;
            default: dir = new int[2];
        }
        int row = headPos[0] + dir[0]; // new position row
        int col = headPos[1] + dir[1]; // new position col
        
        //delete tail first, before push into a new cell
        if (foodSeq<foods.length && calcPosId(foods[foodSeq][0], foods[foodSeq][1]) == calcPosId(row, col)) {
            foodSeq++;
        }
        else {
            set.remove(queue.poll());
        }
        
        if (row<0 || row>=m || col<0 || col>=n) return -1; // hit border
        if (set.contains(calcPosId(row, col))) return -1; // hit its own body
        
        set.add(calcPosId(row, col));
        queue.offer(calcPosId(row, col));
        headPos[0] = row;
        headPos[1] = col;

        return set.size()-1;
    }
    
    public int calcPosId(int x, int y) {
        return x*n+y;
    }
}

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

 如果没有39行，程序会说dir没有initialize,

其实更好的写法应该是

        int rowHead = body.peekFirst() / width;
        int colHead = body.peekFirst() % width;
        switch (direction) {
            case "U" : rowHead--;
                       break;
            case "D" : rowHead++;
                       break;
            case "L" : colHead--;
                       break;
            default :  colHead++;
        }