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Binary Tree Interview Questions — Pattern Guide

A pattern guide to binary tree interview questions: traversals, BFS/DFS, recursion patterns, and the problems that come up most often.

Updated 2026-09MixedTrees

Why trees come up so often

Binary trees test recursion, pointer manipulation, and the ability to split a problem into independent subproblems. Most tree questions reduce to a post-order combination of left and right results.

Core patterns

DFS traversals (pre-order, in-order, post-order) — the backbone of most solutions.
BFS / level-order with a queue — for level-by-level problems.
Bottom-up recursion: solve left and right, then combine (max depth, diameter, is-balanced).
BST property: in-order traversal yields sorted order; prune searches by comparison.

Most-asked problems

Maximum depth of a binary tree (simplest recursion).
Invert / mirror a binary tree.
Lowest common ancestor of two nodes.
Diameter (longest path between any two nodes).
Level-order traversal and its zig-zag variant.
Validate a binary search tree.
Construct a tree from pre-order + in-order traversals.

How to approach any tree question

Decide DFS vs BFS: DFS for depth/ancestor/path questions, BFS for level questions.
Write the base case (null node) first.
Define what the recursive call returns, then combine children.
Consider passing state down (e.g., a running max) or up (returning a computed value).

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