N-ary Tree Preorder Traversal

N-ary Tree Preorder Traversal

What is this problem about?

The N-ary Tree Preorder Traversal problem asks you to traverse an N-ary tree in pre-order: visit the current node first, then recursively visit each child from left to right. Return all node values in this order. This N-ary Tree Preorder Traversal coding problem is a foundational DFS tree traversal exercise.

Why is this asked in interviews?

Amazon and Google ask this as a fundamental tree traversal question. Pre-order traversal is used in tree serialization, file system traversal, and expression tree evaluation. The DFS, stack, and tree interview pattern is directly applied, and the ability to implement it both recursively and iteratively is expected.

Algorithmic pattern used

Recursive DFS: visit current node, then recursively visit each child. Iterative with stack: Push root. While stack is non-empty: pop node, append value to result, push children right-to-left (so left children are processed first — the right child is pushed first and thus processed last).

Example explanation

Tree: root=1, children=[3,2,4]. Node 3's children=[5,6]. Pre-order: 1 → 3 → 5 → 6 → 2 → 4. Iterative: push 1. Pop 1 (value=1), push [4,2,3] (right to left). Pop 3 (value=3), push [6,5]. Pop 5 (value=5). Pop 6 (value=6). Pop 2 (value=2). Pop 4 (value=4). Result: [1,3,5,6,2,4].

Common mistakes candidates make

• Pushing children in left-to-right order in the stack (reverses the traversal order).
• Not handling nodes with no children (just don't push anything for that node).
• Recursive solution: iterating children in wrong order.
• Confusing pre-order with level-order (BFS processes level by level; DFS pre-order processes depth-first).

Interview preparation tip

Pre-order traversal is the "root first" DFS. For iterative implementation, remember the stack reversal trick: to visit children left-to-right, push them right-to-left onto the stack. Practice all traversal orders and their iterative counterparts. N-ary tree traversals generalize directly to file system DFS (pre-order for printing directories before files) and DOM tree traversal (pre-order for processing parent nodes before children).