在采访中设计简单的贪婪蛇游戏的数据结构

Question

This is not a practical question, I just want to discuss and learn data structure design here, I heard it's asked by Google during onsite interview. Please advise me how to improve my design, thanks!

At the beginning I wanted to use a deque to store pairs of x,y coordinates for snake's body parts.

deque<pair<x, y>> snakeBodyParts;

Because it's very easy to push front when snake move - create new coordinate pair as the new head base on old head position and current direction, then pop back to remove the tail. In this way move, eat, check head hit wall are all O(1) operations and easy to implement with deque. But checking if new head position overlap with snakes body will require looping through all body parts' position - O(L) time complexity, L is number of body parts.

To improve it, I thought about putting all coordinates into an unordered_set(C++) or hashset(Java) while still keeping my old deque, it can give me O(1) for checking if head hits body now. But I don't know if it's a good idea because it almost doubles the memory and amount of codes, whenever I add/remove to deque, I need to do it to my hashset.

unordered_set<pair<int, int>, pair_hash> bodyPartsSet;

I also thought about creating my own structure which is like linkedlist, except it points to previous node:

SnakeBodyNode {
    int x;
    int y;
    SnakeBodyNode * prev;
}

Then I also need two pointers pointing at head and tail as well as a direction variable.

SnakeBodyNode * head;
SnakeBodyNode * tail;
char dir;

However I don't see any advantage of this, still need to hash to get O(1) for checking if head hits body..

Is there any flaw in my deque + hash design or any one have better idea to share?

Answer 1

I would just use an unordered_set. Your concerns are:

• Fast insert of new head - this is O(1) for unordered_set.
• Fast deletion of existing tail - this is O(1) for unordered_set.
• No duplicates (checking that head intersects body) - this is guaranteed for unordered_set (it does not allow duplicates).

When inserting a new head, you don't have to do anything special to check that it intersects with the body; you'll get an error if it does.