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 will be at (row + 1, col - 1) and its right child will be at (row + 1, col + 1). Report nodes from top to bottom, sorted by col then by row, then by value if multiple nodes are at the same (row, col).

• The root of a binary tree.

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