DSA interview question

Binary Tree Preorder Traversal

Easy · Data structures · Topics: tree, dfs, recursion

The problem

A file-explorer widget shows a folder tree, and its "export as outline" feature writes each folder before its contents: the folder itself, then everything in its first child, then everything in its second child. For a binary tree that's preorder: root, left subtree, right subtree. Recursion expresses it almost word for word, since visiting a subtree is the same problem on a smaller tree.

Example: [1, null, 2, 3] (1 has right child 2, and 2 has left child 3) → [1, 2, 3].

Function signature

vector<int> preorderTraversal(TreeNode* root);

Test cases

Input (level order)	Output	Explanation
`[1, null, 2, 3]`	`[1, 2, 3]`
`[1, 2, 3, 4, 5, null, 8, null, null, 6, 7, 9]`	`[1, 2, 4, 5, 6, 7, 3, 8, 9]`
`[]`	`[]`	Empty tree
`[1]`	`[1]`
`[5, 4, 6]`	`[5, 4, 6]`

(Confirm: the input uses LeetCode's level-order format with null for missing children; values may repeat.)

How interviewers phrase it

"Given the root of a binary tree, return the preorder traversal of its nodes' values" (LeetCode 144).

Clarifying questions to ask

Recursive or iterative? (Recursive first; the iterative version is a common follow-up.)
Can the tree be empty? (Yes.)
How deep can it be? (Up to 100 nodes on LeetCode, so recursion depth is fine. Very deep trees need an explicit stack.)

Follow-ups to expect

"What makes it preorder?"
"Iterative version?"
"O(1) extra space?"
"Where is preorder used?"
"Complexity?"

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