How to implement a breadth first search to a certain depth?

Question

I understand and can easily implement BFS.

My question is, how can we make this BFS limited to a certain depth? Suppose, I just need to go 10 level deep.

Answer 1

The easy Idea for keeping track of the depth is to add "NULL" to the Queue every time you go a level deep. As soon as you poll a null from the queue, Increase your level counter by 1 and add another 'null' to the Queue. If you get two consecutive nulls you can exit from the loop.

q.offer(user);
q.offer(null);

user.setVisited(true);

while(!q.isEmpty()){

    User userFromQ = q.poll();

    if(userFromQ == null){
        level++;
        q.offer(null);
        if(q.peek()==null)
            break;
        else
            continue;
    }