Binary Tree Vertical Order Traversal

Given a binary tree, implement a function to output the tree's vertical order traversal. Each node should be ordered from top to bottom and from left to right. The result should be a list where each sublist represents a vertical level in the tree. Nodes within each vertical level should be ordered from top to bottom. Use breadth-first search algorithm to implement. Constraints: The number of tree nodes is in the range [0, 1000]. Each node has a unique value. Example Input: Input: 3 / \ 9 20 / \ 15 7 Output: [[9], [3, 15], [20], [7]] Example Input 3