#!/usr/bin/env python3 # -*- coding: utf-8 -*- # author: bt3gl def build_tree(preorder: list[int], inorder: list[int]) -> Optional[Node]: def helper(i_left, i_right, index_map): if i_left > i_right: return None root = TreeNode(preorder.pop(0)) index_here = index_map[root.val] # this order change from postorder root.left = helper(i_left, index_here - 1, index_map) root.right = helper(index_here + 1, i_right, index_map) return root index_map = {value: index for index, value in enumerate(inorder)} return helper(0, len(inorder) - 1, index_map)