解题思路

import java.util.*;

public class Main {
    public static class TreeNode {
        int val = 0;
        TreeNode left = null;
        TreeNode right = null;

        public TreeNode(int val) {
            this.val = val;

        }

    }
    
    public static void main(String[] args) {
        TreeNode root = new TreeNode(1);
        root.right = new TreeNode(2);
        root.left = new TreeNode(3);
        root.left.left = new TreeNode(4);
        root.left.right = new TreeNode(5);
        root.right.left = new TreeNode(6);
        root.right.right = new TreeNode(7);
        
        ArrayList list = print(root);
        printList(list);
    }
    
    public static ArrayList<ArrayList<Integer>> print(TreeNode pRoot) {
        Queue<TreeNode> queue = new LinkedList<>();
        ArrayList<ArrayList<Integer>> ret = new ArrayList<>();
        queue.add(pRoot);
        
        while(!queue.isEmpty()) {
            ArrayList<Integer> list = new ArrayList<>();
            int cnt = queue.size();
            while(cnt-- > 0) {
                TreeNode node = queue.poll();
                if (node == null) {
                    continue;
                } 
                list.add(node.val);
                queue.add(node.left);
                queue.add(node.right);
            }
            
            if (list.size() != 0) {
                ret.add(list);
            }
        }
        
        return ret;
    }
    
    public static void printList(ArrayList list) {
        for (int i = 0;i < list.size();i++) {
            System.out.println(list.get(i) + " ");
        } 
    }
}