DesignSnakeGame.java

package com.fishercoder.solutions;

import java.util.Deque;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.Set;

/**
 * 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)
 */
public class DesignSnakeGame {
    public class SnakeGame {
        private Set<Integer> set;//Use a set to hold all occupied points for the snake body, this is for easy access for the case of eating its own body
        private Deque<Integer> body;//use a queue to hold all occupied points for the snake body as well, this is for easy access to update the tail
        int[][] food;
        int score;
        int foodIndex;
        int width;
        int height;

        /** 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.set = new HashSet();
            set.add(0);//initially at [0][0]
            this.body = new LinkedList<Integer>();
            body.offerLast(0);
            this.food = food;
            this.width = width;
            this.height = height;
        }

        /** 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;

            //compute head
            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++;
            }
            int newHead = rowHead*width+colHead;

            set.remove(body.peekLast());//we'll remove the tail from set for now to see if it hits its tail
            //if it hits the boundary
            if(set.contains(newHead) || rowHead < 0 || colHead < 0 || rowHead >= height || colHead >= width){
                return score = -1;
            }

            //add head for the following two normal cases:
            set.add(newHead);
            body.offerFirst(newHead);

            //normal eat case: 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 since we removed it earlier
                foodIndex++;
                return ++score;
            }


            //normal move case without eating: move head and remove tail
            body.pollLast();
            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);
 */
}