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This is a Medium difficulty Coding & Algorithms question, commonly asked during Technical Screen rounds at Meta.

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This question is commonly asked for Software Engineer candidates at Meta during technical interviews.

Implement BST vertical traversal and list conversion

Quick Overview

This question evaluates proficiency with binary search trees, vertical-order traversal semantics, in-place pointer manipulation for converting a BST into a sorted doubly linked list, and algorithmic complexity analysis within the Coding & Algorithms domain.

Quick Overview

Implement BST vertical traversal and list conversion

Company: Meta

Role: Software Engineer

Category: Coding & Algorithms

Difficulty: Medium

Interview Round: Technical Screen

Given the root of a binary search tree (BST): 1) Implement vertical order traversal: assign the root column 0, left child column −1, right child column +1; return columns from leftmost to rightmost. Within each column, list node values from top to bottom, breaking ties by left-to-right order at the same depth. Provide the algorithm, data structures used, and complexity analysis. 2) Convert the BST into a sorted doubly linked list in-place (non-circular) by reusing nodes (left becomes prev, right becomes next). Return the head of the list. Discuss recursive vs iterative approaches, edge cases (empty tree, single node, skewed tree), and time/space complexity.

Quick Answer: This question evaluates proficiency with binary search trees, vertical-order traversal semantics, in-place pointer manipulation for converting a BST into a sorted doubly linked list, and algorithmic complexity analysis within the Coding & Algorithms domain.

BST Vertical Order Traversal

Given a BST as level-order values, return vertical columns from left to right in breadth-first order within each column.

Constraints

Inputs are Python literals matching the function signature.
Return a deterministic exact-match value.

Examples

Input: ([5,3,7,2,4,6,8],)

Expected Output: [[2], [3], [5, 4, 6], [7], [8]]

Explanation: Balanced BST.

Input: ([],)

Expected Output: []

Explanation: Empty tree.

Input: ([5,3,None,2],)

Expected Output: [[2], [3], [5]]

Explanation: Left-skewed partial tree.

Hints

Choose a representation that makes the requested operation direct.
Handle empty inputs and boundary cases first.

BST to Sorted Doubly Linked List Values

Given a BST as level-order values, return the values in the sorted order that the in-place doubly linked list would contain.

Constraints

Inputs are Python literals matching the function signature.
Return a deterministic exact-match value.

Examples

Input: ([5,3,7,2,4,6,8],)

Expected Output: [2, 3, 4, 5, 6, 7, 8]

Explanation: Sorted in-order list.

Input: ([1],)

Expected Output: [1]

Explanation: Single node.

Input: ([],)

Expected Output: []

Explanation: Empty tree.

Hints

Choose a representation that makes the requested operation direct.
Handle empty inputs and boundary cases first.

Last updated: Jun 27, 2026

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