[Leetcode] 353. Design Snake Game 解题报告

题目：

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)

思路：

本题目的实现方式很多，下面我们着重解释我们的实现方式。非常值得注意的是我们要区分两种状态：一种是已经上一步没有吃掉food的情况，一种是上一步恰好吃掉了food（我们在实现中用flag来表示）。当上一步已经吃掉了一个food的时候，下一步蛇的身体会自动增长1。注意到这一点，我们设计的数据结构如下：

1）队列：用来模拟蛇，队列末尾是蛇头，队列首位是蛇尾；

2）哈希表：用来表示当前蛇所占用的位置；

3）位置向量：用来表示一系列食物的位置；

4）整数标志：表示网格大小，当前位置，当前已经吃到的食物的位置，以及状态标志flag等。

代码：

class SnakeGame {
public:
    /** 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]. */
    SnakeGame(int width, int height, vector<pair<int, int>> food) {
        w = width, h = height, x = 0, y = 0, cur = 0, flag = 0, fd = food;
        que.push(0);
        hash[0] = true;
    }
    
    /** 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) {
        if(direction == "U")    --x;  
        if(direction == "L")    --y;  
        if(direction == "R")    ++y;  
        if(direction == "D")    ++x;  
        if(flag == 0) {  
            int val = que.front();      // remove the tail of the snake   
            hash.erase(val);  
            que.pop();  
        }  
        if(x < 0 || y  < 0 || x >= h || y >= w || hash.count(x * w + y)) {
            return -1;  
        }
        que.push(x*w+y);                // add the head to the snake
        hash[x*w+y] = true;  
        flag = 0;                       // reset the flag
        if(x == fd[cur].first && y == fd[cur].second) {
            flag = 1;
            ++cur;
        }
        return que.size() - 1 + flag;
    }
private:
    queue<int> que;                     // the snake
    unordered_map<int, bool> hash;      // records the blocks that the snake occupies
    int w, h, x, y, cur, flag;          // flag means whether the current food has been eaten
    vector<pair<int, int>> fd;          // the food positions
};

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