一. 思路

核心思想是采用广度优先遍历算法，遍历二叉树的每一层，用LinkedList存储每一层的节点。遍历LinkedList就是遍历二叉树的每一层。

二. 代码

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

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

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

    }

}
*/
public class Solution {
    public ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
       LinkedList<TreeNode> q = new LinkedList<>();
        ArrayList<ArrayList<Integer>> res = new ArrayList<>();
        boolean flag = true;
        q.add(pRoot);

        while (!q.isEmpty()) {
            int size = q.size();
            ArrayList<Integer> list = new ArrayList<Integer> ();
            for (int i = 0; i < size; i++) {
                TreeNode node = q.poll();
                if(node == null) continue;
                if(flag){
                    list.add(node.val);
                } else {
                    list.add(0, node.val);
                }
                q.add(node.left);
                q.add(node.right);
            }
            if (list.size() != 0) res.add(list);
            flag = !flag;
        }
        return res;
    }

}