102. Binary Tree Level Order Traversal

solution.py

# 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 traverse_tree(self, node: Optional[TreeNode], depth: int, levels: list[list[int]]):
        if node is None:
            return

        if len(levels) == depth:
            levels.append(list())

        levels[depth].append(node.val)
        self.traverse_tree(node.left, depth + 1, levels)
        self.traverse_tree(node.right, depth + 1, levels)

    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
        levels = list()
        self.traverse_tree(root, 0, levels)
        return levels