二叉树按层遍历，利用队列，和last与nlast两个变量实现，last为本层中的最后一个节点，nlast为下一层的最后一个节点，每次从队列中出队元素，加入当前层的集合中，然后先将它的左子树入队（不为空的话），将nlast指向它，再将它的右子树入队（不为空的话），将nlast指向它，最后如果last跟它指向同一个节点，表示当前层已经到最后一个元素了，令last = nlast，将当前层集合加入结果集合中，建立新的层集合，如此循环，直到queue为空：

import java.util.ArrayDeque;
import java.util.ArrayList;

public class PrintBinaryTree {

    public static void main(String[] args) {
        TreeNode node1 = new TreeNode(3);
        TreeNode node2 = new TreeNode(4);
        TreeNode node3 = new TreeNode(5);
        TreeNode node4 = new TreeNode(6);
        TreeNode node5 = new TreeNode(7);
        TreeNode node6 = new TreeNode(8);
        node1.left = node2;
        node1.right = node3;
        node2.left = node4;
        node3.left = node5;
        node3.right = node6;
        int[][] res = printTree(node1);
        for(int i = 0; i < res.length; i++) {
            for(int j = 0; j < res[i].length; j++) {
                System.out.print(res[i][j] + " ");
            }
            System.out.println();
        }
        
    }

    

    public static int[][] printTree(TreeNode root) {
        ArrayDeque<TreeNode> aq = new ArrayDeque<>();
        ArrayList<ArrayList<Integer>> res = new ArrayList<>();
        ArrayList<Integer> index = new ArrayList<>();
        TreeNode last = null;
        TreeNode nlast = null;
        TreeNode temp;
        if(root == null)
            return null;
        else {
            last = root;
            aq.addLast(root);
        }
            
        while(aq.size() != 0) {

            temp = aq.removeFirst();
            index.add(temp.val);
            if(temp.left != null) {
                aq.addLast(temp.left);
                nlast = temp.left;
            }
            if(temp.right != null) {
                aq.addLast(temp.right);
                nlast = temp.right;
            }
            if(temp == last) {
                res.add(index);
                index = new ArrayList<>();
                last = nlast;
            }

            
        }

        int result[][] = new int[res.size()][];
        for(int i = 0; i < res.size(); i++) {
            result[i] = new int[res.get(i).size()];
            for(int j = 0; j < result[i].length; j++) {
                result[i][j] = res.get(i).get(j);
            }
        }
        return result;
    }
}

class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;
    public TreeNode(int val) {
        this.val = val;
    }
}

结果

3 
4 5 
6 7 8