def levelOrder(self, root):
    """
    :type root: TreeNode
    :rtype: List[List[int]]
    """
    stack, output = [], []
    if root:
    stack.append(root)
    output.append([root.val])
    while stack:
        lst, lst2 = [], []
        while stack:
            node = stack.pop(0)
            if node.left:
                lst.append(node.left)
                lst2.append(node.left.val)
            if node.right:
                lst.append(node.right)
                lst2.append(node.right.val)
        if lst:
            output.append(lst2)
            stack = lst
    return output