## Saturday, June 4, 2016

### LeetCode 353 - Design Snake Game

http://www.cnblogs.com/grandyang/p/5558033.html
Design a Snake game that is played on a device with screen size = width x heightPlay 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)
```

```    SnakeGame(int width, int height, vector<pair<int, int>> food) {
this->width = width;
this->height = height;
this->food = food;
score = 0;
pos.push_back({0, 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. */
int move(string direction) {
auto head = pos.front(), tail = pos.back();
pos.pop_back();
else if (direction == "L") --head.second;
else if (direction == "R") ++head.second;
else if (direction == "D") ++head.first;
return -1;
}
if (!food.empty() && head == food.front()) {
food.erase(food.begin());
pos.push_back(tail);
++score;
}
return score;
}

private:
int width, height, score;
vector<pair<int, int>> food, pos;```
X.
http://dartmooryao.blogspot.com/2016/06/leetcode-353-design-snake-game.html
In the first try, I used a char matrix, and get memory space exceed error.
So I changed it to a linkedlist.

The idea is to use a linkedList to represents the body of snake, and use a hashset to represents the position of snake's body. So we can get constant time to check whether the next position is valid.

Also note that the snake can move to it's own tail. So it means that we have to delete the tail position from the hashset before we check the valid.
public class SnakeGame {

Map<String, int[]> map;
int idx = 0;
int[][] food;
int width;
int height;
Set<String> posi = new HashSet<>();

/** 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.food = food;
this.width = width;
this.height = height;
map = createMap();
}

/** 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 = map.get(direction);
int[] tail = snake.get(snake.size()-1);
posi.remove(tail[0]+"_"+tail[1]);

String posiKey = nextR+"_"+nextC;
if(nextR<0 || nextC<0 || nextR>=height || nextC>=width || posi.contains(posiKey)){ return -1; }

if(idx<food.length && nextR==food[idx][0] && nextC==food[idx][1]){
idx++;
}else{
snake.removeLast(); // posi.remove(tail)?
}
return snake.size()-1;
}

private Map<String, int[]> createMap(){
Map<String, int[]> map = new HashMap<>();
map.put("U", new int[]{-1, 0});
map.put("L", new int[]{0, -1});
map.put("R", new int[]{0, 1});
map.put("D", new int[]{1, 0});
return map;
}
}
https://leetcode.com/discuss/106258/accepted-java-solution
private int width, height; int[][] food; private int score, idx; private Deque<Integer> snake; private Set<Integer> body; public SnakeGame(int width, int height, int[][] food) { this.width = width; this.height = height; this.food = food; score = 0; idx = 0; snake = new LinkedList<>(); snake.addLast(0); body = new HashSet<>(); body.add(0); } public int move(String direction) { // first of all, get head int h = snake.getFirst() / width, w = snake.getFirst() % width; if (direction.equals("U")) h--; else if (direction.equals("D")) h++; else if (direction.equals("L")) w--; else w++; if (w < 0 || w == width || h < 0 || h == height) return -1; int head = h*width + w; snake.addFirst(head); // move head if (idx < food.length && h == food[idx][0] && w == food[idx][1]) { score++; // and still keep the tail idx++; } else body.remove(snake.removeLast()); // move tail if (!body.add(head)) return -1; return score; }
https://leetcode.com/discuss/106217/java-solution-of-snakegame-276ms
// use another hashmap to speed the contains