Binary Tree Vertical Order Traversal

Problem Return the vertical order traversal of a binary tree: group nodes by their horizontal column (root is column 0, a left child is column − 1, a right child is column + 1), listing columns left to right and, within a column, top to bottom.

Input / Output

• Input: root of a binary tree.
• Output: list of columns, each a list of node values top-to-bottom.

Constraints

• 0 ≤ nodes ≤ 100.
• Maintain stable top-to-bottom (and left-to-right within a cell) order.

Example

• Input [3,9,20,null,null,15,7] → [[9],[3,15],[20],[7]].