from collections import deque

queue = deque([root]) if root else deque()
levels = []

while queue:
    level = []
    for _ in range(len(queue)):
        node = queue.popleft()
        level.append(node.val)
        if node.left:
            queue.append(node.left)
        if node.right:
            queue.append(node.right)
    levels.append(level)