Solution.java

import java.util.*;

public class Solution
{
    public List<List<Integer>> levelOrder(TreeNode root)
    {
        List<List<Integer>> result = new ArrayList<>();
        MyQueue<TreeNode> queue = new MyQueue<>();
        queue.addIfNotNull(root);
        while (!queue.isEmpty())
        {
            MyQueue<TreeNode> nextLevel = new MyQueue<>();
            List<Integer> currentLevel = new ArrayList<>();
            while (!queue.isEmpty())
            {
                TreeNode node = queue.poll();
                currentLevel.add(node.val);
                nextLevel.addIfNotNull(node.left);
                nextLevel.addIfNotNull(node.right);
            }
            result.add(currentLevel);
            queue = nextLevel;
        }
        return result;
    }

    public class MyQueue<E> extends LinkedList<E>
    {
        private static final long serialVersionUID = 1L;

        public void addIfNotNull(E e)
        {
            if (e != null)
            {
                add(e);
            }
        }
    }
}