一. 思路

采用层次遍历二叉树，用队列存储每一层的节点。要注意加入根节点时要判空，空则直接返回空数组出去。

二. 代码

import java.util.*;


/*
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

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

    }

}
*/
public class Solution {
    ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
     Queue<TreeNode> queue = new LinkedList<>();
     ArrayList<ArrayList<Integer>> result = new ArrayList<>();

     if (pRoot == null) {
         return result;
     }
     queue.offer(pRoot);

     while (!queue.isEmpty()) {
         int size = queue.size();
         ArrayList<Integer> list = new ArrayList<>();
         for (int i = 0; i < size; i++) {
             TreeNode node = queue.poll();

             list.add(node.val);
             if (node.left != null) {
                 queue.offer(node.left);
             } 
             if (node.right != null) {
                 queue.offer(node.right);
             }
         }
         result.add(list);
     }

     return result;
    }

}