from collections import deque
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None
def level_order_tree(root, result):
    if not root:
        return
    queue = deque([root])
    while queue:
        node = queue.popleft()
        # do somethings
        result.append(node.val)
        if node.left:
            queue.append(node.left)
        if node.right:
            queue.append(node.right)
    return result