Postorder Traversal — Left, Right, Root

Postorder finishes both subtrees before acting at the root—natural for subtree sums, deletion, and bottom-up DP on trees.

Try this in Python

from __future__ import annotations
from dataclasses import dataclass


@dataclass
class TreeNode:
    val: int
    left: TreeNode | None = None
    right: TreeNode | None = None


def postorder(root: TreeNode | None) -> list[int]:
    if not root:
        return []
    return postorder(root.left) + postorder(root.right) + [root.val]

r = TreeNode(1, TreeNode(2), TreeNode(3))
print(postorder(r))

Key takeaways

• Two-stack trick can reverse a preorder to postorder iteratively—good to sketch once.
• Many divide-and-conquer proofs on trees are post-order shaped.
• Deletion often frees children first conceptually.