def (self, root: TreeNode) -> int:
	if root is None:
            return 0
    xx = 1
    queue = [root]
    while queue:
        cache = []
        
        for q in queue:
            if q.left is None and q.right is None:
            	# 此处是离根最近的一个叶子结点
                pass
            if q.left is not None:
                cache.append(q.left)
            if q.right is not None:
                cache.append(q.right)
        xx += 1 # 此时xx是cache中结点所在的层数
        queue = cache