参考代码:

class TreeNode:
    def __init__(self, val):
        self.val = val
        self.left = None
        self.right = None


class Solution:
    def isComplete(self, root):
        if root is None:
            return True
        queue = [root]
        index = 0
        while index < len(queue):
            if queue[index] is not None:
                queue.append(queue[index].left)
                queue.append(queue[index].right)
            index += 1
        while queue[-1] is None:
            queue.pop()
        for q in queue:
            if q is None:
                return False
        return True