Binary Tree Level Order Traversal

lcpy gen -n 102   # by problem number
lcpy gen -s binary_tree_level_order_traversal   # by problem name

Input: root = [3,9,20,null,null,15,7]
Output: [[3],[9,20],[15,7]]

Input: root = [1]
Output: [[1]]

Input: root = []
Output: []

from collections import deque

from leetcode_py import TreeNode


class Solution:
    # Time: O(n)
    # Space: O(w) where w is max width of tree
    def level_order(self, root: TreeNode[int] | None) -> list[list[int]]:
        if not root:
            return []

        result = []
        queue = deque([root])

        while queue:
            level_size = len(queue)
            level = []

            for _ in range(level_size):
                node = queue.popleft()
                level.append(node.val)

                if node.left:
                    queue.append(node.left)
                if node.right:
                    queue.append(node.right)

            result.append(level)

        return result