What are the tree traversal methods?

Q: What are the tree traversal methods?

**Target Audience:** Beginners preparing for coding interviews; no prior knowledge of trees assumed. **Difficulty:** Easy **Language:** Python 3 > **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. Every code block in this Python version was run, and the **Verified** section quotes the real output. ## 1. Why This Even Matters — A Story First Imagine you have just moved into a big old house with rooms that branch off rooms. You want to check every room exactly once for a lost key. There are only two sensible plans. Plan one: walk down a corridor as far as it goes, then back up and try the next door (going **deep** first). Plan two: check every room on the ground floor, then every room on the first floor, then the second (going **wide** first, floor by floor). A tree traversal is the same choice for a tree of data. Every tree algorithm you will meet in an interview

The problem

Question presented to candidate: "Walk me through the ways you can traverse a binary tree. Then, in Python, write preorder, inorder and postorder recursively, write inorder iteratively with a stack, and tell me when you would pick each order."

What a strong answer should cover:

The two families: depth-first (preorder, inorder, postorder) and breadth-first (level order with a queue), and that the three DFS orders differ only in when the node is visited.
The output of each order on a small example, and that inorder of a BST is sorted.
Typical uses: preorder to copy or serialise, inorder for BST problems, postorder when a node needs its children's results, level order for per-level questions.
O(n) time for all; O(h) space for DFS (call stack or explicit stack) and O(w) for BFS.
Python specifics: the default recursion limit is 1000, so a deep chain raises RecursionError and the iterative version matters; a list is the stack and collections.deque is the queue (list.pop(0) is O(n)); never use a mutable default argument like out=[].

Code / implementation expected: Yes — the three recursive orders in Python plus at least one iterative version with a list as the stack.

traversal
trees

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Beginners preparing for coding interviews; no prior knowledge of trees assumed. Difficulty: Easy Language: Python 3

How to read this doc: Every idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact words an interviewer expects. Every code block in this Python version was run, and the Verified section quotes the real output.

1. Why This Even Matters — A Story First

Imagine you have just moved into a big old house with rooms that branch off rooms. You want to check every room exactly once for a lost key. There are only two sensible plans. Plan one: walk down a corridor as far as it goes, then back up and try the next door (going deep first). Plan two: check every room on the ground floor, then every room on the first floor, then the second (going wide first, floor by floor).

A tree

The solution is hidden so you can try first

15 min read · 9 sections

Python code

Edit the code and run it here. Each printed line with an expected value becomes a test.

PythonRecursive preorder, inorder, postorder and level order

Practice

PythonIterative traversals with an explicit stack, a BST and a 100000-node chain