Implement postorder traversal

Easy

≈ 10 minute session

Summary

Implement postorder (left-right-root) traversal for a binary tree, returning node values in a list.

Problem statement

Given the root of a binary tree, implement a function postorder(root) that returns a list of node values visited in postorder (left, right, root). Use the provided Node class. Return an empty list for an empty tree.

Task

Write postorder(root) to traverse left subtree, then right subtree, then visit root, returning a list of the values.

Examples

Three-node tree

Input

postorder(Node(1, Node(2), Node(3)))

Output

[2, 3, 1]

Explanation

Visit left 2, right 3, then root 1.

Input format

A single argument: root (Node or None).

Output format

A list of integers representing node values in postorder order.

Constraints

Nodes up to 10^4. Node values are integers. Use recursion or iteration. Return list, do not print.

Samples

Sample input 0

postorder(None)

Sample output 0

[]

Explanation 0

Empty tree returns empty list.

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