Binary Tree Vertical Order Traversal

Given the root of a binary tree, return the vertical order traversal of its nodes' values. For each node at position (row, col), its left child is at (row + 1, col - 1) and right child at (row + 1, col + 1). The root starts at column 0. Return all nodes' values grouped by column from left to right. Within each column, nodes are ordered from top to bottom. If two nodes are in the same row and column, order them from left to right.