102-binary-tree-level-order-traversal.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

from collections import defaultdict


class Solution:
    def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
        if not root:
            return []

        ret = []

        def helper(node, level):
            if len(ret) == level:
                ret.append([])

            ret[level].append(node.val)

            if node.left:
                helper(node.left, level+1)
            if node.right:
                helper(node.right, level+1)

        helper(root, 0)

        return ret