# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

import Queue

class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """

        if not root:
            return []

        q = Queue.Queue()

        q.put(root)
        result = []

        while not q.empty():
            level = []
            qsize = q.qsize()
            for _ in range(qsize):
                current = q.get()
                level.append(current.val)
                if current.left:
                    q.put(current.left)
                if current.right:
                    q.put(current.right)
            result.append(level)
        return result