107. Binary-Tree-Level-Order-Traversal-II

# 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 levelOrderBottom(self, root: Optional[TreeNode]) -> List[List[int]]:
        if not root:
            return []
        queue = deque([root])
        res = []
        while queue:
            row = []
            n = len(queue)
            for _ in range(n):
                node = queue.popleft()
                row.append(node.val)
                if node.left:
                    queue.append(node.left)
                if node.right:
                    queue.append(node.right)
            res.append(row)

        res.reverse()
        return res