# Python3

from collections import deque

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

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

        # approach: use a queue for level order traversal

        if not root:
            return []

        result = []
        queue = deque([root])
        q_size = 1

        while q_size:
            level_result = []
            for i in range(q_size):
                node = queue.popleft()
                level_result.append(node.val)
                if node.left:
                    queue.append(node.left)
                if node.right:
                    queue.append(node.right)

            result.append(level_result)
            q_size = len(queue)

        return result