Example 1
Input
inorder = [9, 3, 15, 20, 7], postorder = [9, 15, 7, 20, 3]
Output
[3, 9, 20, 15, 7]
The reconstructed tree has root 3, left child 9, and right subtree with root 20 and children 15 and 7. Preorder (root-left-right) is [3,9,20,15,7].
Full lesson preview
Construct a tree from inorder and postorder
Reconstruct a binary tree from its inorder and postorder traversals and return its preorder traversal.
Python practice25 minTrees & Binary TreesAdvancedLast updated March 29, 2026
Problem statement
You are given two integer arrays inorder and postorder where inorder is the inorder traversal of a binary tree and postorder is the postorder traversal of the same tree. All values are unique. Reconstruct the binary tree and return its preorder traversal as a list of integers (root-left-right).
Return format: a Python list representing the preorder traversal. Example: [1, 2, 3]. If the tree is empty (both lists empty), return an empty list [].
Task
Given inorder and postorder traversal arrays (with unique values), rebuild the original binary tree and output a list of node values in preorder (root-left-right).
Examples
Input format
Two lists of integers: inorder and postorder (both have the same length, values are unique). Example call: build_tree([9,3,15,20,7], [9,15,7,20,3])
Output format
A list of integers representing the preorder traversal of the reconstructed tree. Example: [3, 9, 20, 15, 7]
Constraints
0 <= len(inorder) == len(postorder) <= 1000; All integers in inorder and postorder are unique; The inputs always represent a valid binary tree.
Samples
Sample 1
Input
inorder = [2, 1, 3], postorder = [2, 3, 1]
Output
[1, 2, 3]
Reconstructed tree root 1 with left 2 and right 3 -> preorder [1,2,3].