Untitled

from collections import deque

class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        q, ans = deque([root]), []
        while q:
            size, level = len(q), []
            for _ in xrange(size):
                node = q.popleft()
                if node:
                    level.append(node.val)
                    q.extend([node.left, node.right])
            if level:
                ans.append(level)
        return ans