# 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]]:

res = []

q = collections.deque()

q.append(root)

while q:

lengthQ = len(q)

level = []

for i in range(lengthQ):

node = q.popleft()

if node:

level.append(node.val)

q.append(node.left)

q.append(node.right)

if level:

res.append(level)

return res