# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right
class Solution:
def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
q = deque()
res = []
q.append(root)
while q:
current = len(q)
level = []
for _ in range(current):
node = q.popleft()
if node is not None:
q.append(node.left)
q.append(node.right)
level.append(node.val)
if len(level):
res.append(level)
return res