from collections import deque

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right
def is_complete_tree(root):
    if root is None:
        return True
    q = deque()
    q.append(root)
    has_null = False

    while q:
        node = q.popleft()

    if node is None:
        has_null = True
    else:
        if has_null:
            return False
        q.append(node.left)
        q.append(node.right)

    return True