Mock Coding Interview: Vertical Traversal of a Binary Tree
A full mock coding interview walkthrough covering LeetCode 987: Vertical Order Traversal of a Binary Tree, one of the trickier medium-hard tree problems that comes up in technical interviews at companies like Google, Meta, and Amazon.
The interview starts with a warm-up problem — level-order (BFS) traversal of a binary tree — before moving into the main challenge. The candidate works through the vertical traversal problem step by step: augmenting each node with horizontal offset and depth values, partitioning nodes into vertical columns using a dictionary, and handling the subtle ordering rules when nodes share the same position. You'll see the real thought process behind choosing pre-order DFS versus level-order BFS, why pre-order alone doesn't guarantee correct depth ordering, and how switching to BFS or adding a depth-based sort fixes the edge cases most candidates miss.
0:00 Introduction and warm-up: level-order traversal of a binary tree
2:19 Main problem: vertical traversal of a binary tree
3:56 Approach discussion: augmenting nodes with horizontal and depth values
6:40 Connecting BFS from warm-up to the vertical traversal approach
8:21 Coding the solution in Python (DFS with dictionary partitioning)
14:09 Building a test case and debugging the tree structure
19:25 Edge case: pre-order traversal doesn't guarantee correct depth order
22:35 Fixing the ordering with depth-based tuple sorting
27:41 Follow-up discussion: replacing DFS with level-order BFS
30:30 Interviewer feedback and tips on clarifying questions
Problem link:
LeetCode 987 — Vertical Order Traversal of a Binary Tree: https://leetcode.com/problems/vertica...