Tree Algorithms: Complete Interview Guide

Tree Algorithms: Complete Interview Guide

Tree problems appear in approximately 20% of coding interviews and are especially common at Google and Meta. The key to mastering tree problems is understanding traversal patterns and recognizing when to use DFS vs BFS.

Three traversal patterns — in-order, pre-order, and post-order DFS — plus level-order BFS solve the majority of tree interview problems.

Traversal Patterns

Traversal	Order	Use Case	Implementation
In-Order	Left → Root → Right	BST sorted output	Recursive or stack
Pre-Order	Root → Left → Right	Serialize tree, copy tree	Recursive or stack
Post-Order	Left → Right → Root	Delete tree, evaluate expression	Recursive
Level-Order (BFS)	Level by level	Shortest path, level aggregation	Queue

Must-Know Tree Problems

• Maximum depth of binary tree (DFS, O(n))
• Validate BST (in-order traversal with bounds)
• Lowest Common Ancestor (recursive DFS)
• Level order traversal (BFS with queue)
• Serialize and deserialize binary tree (pre-order + null markers)

Trees are foundational for graph algorithms and dynamic programming on trees. Practice with AissenceAI's coding copilot.