N-ary Tree Postorder Traversal

N-ary Tree Postorder Traversal

What is this problem about?

The N-ary Tree Postorder Traversal problem asks you to traverse an N-ary tree in post-order: first recursively visit all children from left to right, then visit the current node. Return all node values in this traversal order. This N-ary Tree Postorder Traversal coding problem tests both recursive and iterative DFS implementations on multi-child trees.

Why is this asked in interviews?

Amazon, Google, and Bloomberg ask this to test fundamental tree traversal mastery on N-ary trees. It validates that candidates can implement post-order DFS cleanly — recursively and iteratively — for trees with arbitrary numbers of children. The DFS, stack, and tree interview pattern is the core.

Algorithmic pattern used

Recursive DFS (simplest): For each node, recursively traverse all children left to right, then append the node's value.

Iterative with stack: Push root. For each popped node, prepend its value to result (or append and reverse at end), then push children left to right. The prepend approach works because we want children processed before parent.

Example explanation

Tree: root=1, children=[3,2,4]. Node 3's children=[5,6]. Recursive: postorder(3) → postorder(5)=[5], postorder(6)=[6], then 3 → [5,6,3]. postorder(2)=[2]. postorder(4)=[4]. Then root: [5,6,3,2,4,1]. Result: [5, 6, 3, 2, 4, 1].

Common mistakes candidates make

• Processing the node before children (that's pre-order, not post-order).
• For iterative: pushing children in the wrong order (need left-to-right for left-to-right post-order).
• Confusing the prepend-and-reverse trick with regular stack order.
• Not handling leaf nodes (no children) as the base case.

Interview preparation tip

Knowing both recursive and iterative implementations of tree traversals is essential. The iterative post-order trick — push to stack, append to result, push children left-to-right, then reverse the result — mirrors the pre-order reversal trick. Practice all three traversals (pre, in for binary, post) both recursively and iteratively for binary and N-ary trees. Being able to convert between them instantly demonstrates strong tree manipulation skills.