353. Design Snake Game

        /*
         * 353. Design Snake Game
         * 2016-7-12 by Mingyang
         */
        class SnakeGame {
            //2D position info is encoded to 1D and stored as two copies 
            Set<Integer> set; // this copy is good for fast loop-up for eating body case
            Deque<Integer> body; // this copy is good for updating tail
            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<>();
                set.add(0); //intially at [0][0]
                body = new LinkedList<>();
                body.offerLast(0);
            }              
            public int move(String direction) {
                //case 0: game already over: do nothing
                if (score == -1) {
                    return -1;
                }                
                // compute new 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 head = rowHead * width + colHead;
                
                //case 1: out of boundary or eating body
                set.remove(body.peekLast()); // new head is legal to be in old tail's position, remove from set temporarily 
                if (rowHead < 0 || rowHead == height || colHead < 0 || colHead == width || set.contains(head)) {
                    return score = -1;
                }                
                // add head for case2 and case3
                set.add(head); 
                body.offerFirst(head);                
                //case2: 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;
                }                
                //case3: normal move, remove tail, add head
                body.pollLast();
                return score;                
            }
        }