Tree Questions at Josh Technology: What to Expect

Tree questions make up a significant portion of the technical interview at Josh Technology, with 10 out of 36 total problems dedicated to this structure. This emphasis reflects the data model's prevalence in real-world systems, from representing hierarchical data like file directories and organizational charts to forming the backbone of databases (e.g., B-trees) and search algorithms. Mastering trees demonstrates your ability to handle recursive logic, manage complex state, and traverse non-linear data—core skills for backend and full-stack roles at the company.

What to Expect — Types of Problems

The tree problems at Josh Technology typically focus on binary trees and binary search trees (BSTs). You can expect questions in these categories:

• Traversal Problems: Implementing and modifying standard traversals (in-order, pre-order, post-order, level-order) to extract data, validate properties, or find paths.
• Property Validation: Checking if a tree is a valid BST, balanced, symmetric, or if two trees are identical/isomorphic.
• Path & Sum Problems: Finding maximum/minimum path sums, counting paths that sum to a target, or finding the lowest common ancestor (LCA) of two nodes.
• Construction & Conversion: Building a tree from given traversal outputs (like inorder and preorder) or converting a tree into a linked list or another structure.
• BST Operations: Searching, inserting, deleting nodes, and leveraging BST properties for efficient solutions.

How to Prepare — Study Tips with One Code Example

Start by solidifying your understanding of fundamental traversals using both recursion and iteration. Then, practice applying these patterns to solve the problem categories above. A key strategy is to master the Depth-First Search (DFS) recursive pattern, which forms the basis for most solutions. Break each problem into what needs to be computed at the current node, what needs to be passed to the children, and what needs to be returned up the call stack.

For example, a common task is finding the maximum depth of a binary tree. The recursive logic is: the depth at the current node is 1 (for the node itself) plus the maximum of the depths of its left and right subtrees.

def maxDepth(root):
    if not root:
        return 0
    left_depth = maxDepth(root.left)
    right_depth = maxDepth(root.right)
    return 1 + max(left_depth, right_depth)

Recommended Practice Order

1. Master Fundamentals: Implement all tree traversals recursively and iteratively. Solve basic problems like finding depth, checking for symmetry, and inverting a tree.
2. Tackle Path & Sum Problems: Move to problems involving path sums and finding the lowest common ancestor. These build directly on traversal skills.
3. Focus on BST Properties: Solve validation, search, insertion, and deletion problems. Understand how inorder traversal yields a sorted sequence.
4. Handle Complex Construction: Practice building trees from traversal arrays and converting between tree structures.
5. Combine Concepts: Finally, attempt problems that mix concepts, like finding the largest BST in a binary tree or serializing/deserializing a tree.

Practice Tree at Josh Technology