Binary Tree Level Order Traversal

def levelOrder(root):
    res = []
    q = collections.deque()
    if root:
        q.append(root)
        
    while q:
        level = []
        for i in range(len(q)):
            node = q.popleft()
            level.append(node.val)
            if node.left:
                q.append(node.left)
            if node.right:
                q.append(node.right)
        res.append(level)
        
    return res