以下函数 check() 用于判断一棵二叉树是否为

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 check(root):
    if not root:
        return True
    q = deque()
    q.append(root)
    has_null = False
    while q:
        cur = q.popleft()
        if not cur:
            has_null = True
        else:
            if has_null:
                return False
            q.append(cur.left)
            q.append(cur.right)
    return True