Topic

Binary Trees interview questions

All 32 Binary Trees problems from the curated set, easiest first — a core topic for 4 of the 29 engineering roles.

Easy: 11
Medium: 16
Hard: 5
Sheets: 3

Watch the Binary Trees pattern solved step by step

What Binary Trees is

Updated 2026-09-09

A binary tree is a branching data structure that starts at a single top node called the root, like an upside-down family tree. Every node holds a piece of data and can branch out to at most two children below it, known as the left child and the right child. Because there is no ordering rule about which values go left or right, finding a specific item can require checking every single node in the entire tree.

When to reach for it

Reach for binary trees when problems present hierarchical data with left and right child pointers. Questions asking for tree height, maximum depth, path sums from root to leaf, diameter, lowest common ancestor, or checking whether two trees are mirror reflections of each other all signal binary tree traversals. Any problem asking to inspect or reconstruct a tree layer by layer or path by path belongs here.

How to think about it

Think recursively by focusing on what a single node must do. If the current node is null, return the base answer immediately. Otherwise, ask the left child for its result, ask the right child for its result, and combine both answers with the current node value before returning up to the parent. For horizontal scans, use a queue to read nodes layer by layer, measuring the queue length at the start of each layer to group nodes by depth.

What each operation costs

Operation	Time
traverse all nodes using recursion or queue	O(n)
search for an arbitrary value in an unordered tree	O(n)
call stack memory on balanced tree	O(log n)
call stack memory on skewed tree	O(n)

What usually goes wrong

Dereferencing left or right child pointers without checking if the current node is null, throwing null pointer errors on empty trees or leaf nodes.
Defining a leaf node incorrectly by stopping when either child is null instead of checking that both left and right children are simultaneously null.
Computing tree diameter by taking left height plus right height inside a recursive helper without updating a global maximum across every visited node.

Every Binary Trees problem, easiest first

Roles that need Binary Trees

If you are targeting one of these, Binary Trees sits early in your path rather than being optional.

SDE / Backend Engineer Graphics Engineer Compiler / PL Engineer Blockchain / Web3 Engineer

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Binary Trees interview questions, answered

How many Binary Trees problems should I solve for interviews?

32 curated Binary Trees problems cover the patterns interviews repeat: 11 easy, 16 medium and 5 hard. They are drawn from 3 widely used sheets, deduplicated, and ordered easiest first.

Is Binary Trees actually asked in coding interviews?

Yes, though how much depends on the role. Binary Trees is a core topic for 4 of the 29 engineering roles tracked here, including SDE / Backend Engineer, Graphics Engineer, Compiler / PL Engineer. For other roles it is lower frequency and belongs later in a study plan.

Which Binary Trees problem should I start with?

Start with Binary Tree Inorder Traversal (Easy). The list on this page is ordered easiest first for that reason, so working top to bottom builds the pattern before the harder variations arrive.