Binary Tree Postorder Traversal

Input: root = [1,null,2,3]

Output: [3,2,1]

The tree has structure: 1 -> right: 2 -> left: 3. Postorder visits 3 first (leftmost leaf), then 2, then 1 (root).

Input: root = [1,2,3,4,5,null,8,null,null,6,7,9]

Output: [4,6,7,5,2,9,8,3,1]

Visit all left descendants first, then right descendants, then roots - working bottom-up.

Input: root = []

Output: []

Empty tree returns empty list.

Input: root = [1]

Output: [1]

Single node tree returns just that node.

Problem