Binary Tree Vertical Order Traversal

Given the root of a binary tree, return its vertical order traversal. Definitions: The root has column index 0. Left child column = parent column -1; right child column = parent column +1. Output columns from left to right (in increasing column index). Within the same column, output nodes from top to bottom; if multiple nodes share the same row and column, output them from left to right. Return a 2D list ans where each inner list contains values in one column from leftmost to rightmost. Constraints: Number of nodes n: 0 <= n <= 10^4 Example Input (level-order, null for empty): [3,9,20,null,null,15,7] Output: [[9],[3,15],[20],[7]] Example Input [3,9,20,null,null,15,7] Output [[9],[3,15],[20],[7]]