Binary Tree Vertical Order Traversal

Problem

Return the vertical order traversal of a binary tree's nodes (column by column, top to bottom, left to right).

Example

Input: [3,9,20,null,null,15,7]
Output: [[9],[3,15],[20],[7]]

Constraints

• 0 ≤ nodes ≤ 100

Approach

BFS tracking a column index per node; bucket by column. Maintain stable top-to-bottom order.