ConvertTree.java

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;

public class ConvertTree {

    public static void main(String[] args){
        Node root = new Node();
        List<List<Node>> lists = convert(root);
    }

    static List<List<Node>> convert(Node root){
        if(root == null) return null;

        List<List<Node>> result = new ArrayList<>();

        LinkedList<Node> current = new LinkedList<>();

        current.add(root);

        while(current.size() > 0){
            result.add(current);
            LinkedList<Node> parents = current;
            current = new LinkedList<>();
            for(Node parent : parents){
                if(parent.left != null){
                    current.add(parent.left);
                }
                if(parent.right != null){
                    current.add(parent.right);
                }
            }
        }

        return result;
    }

}

class Node {
    Node left;
    Node right;
}