This is one of Meta's most frequently asked tree problems, testing BFS with column tracking.
You're given a binary tree. Return its vertical order traversal as a list of lists. Column is the root. Left children go to column , right children to column . Within the same column, nodes appear top to bottom, left to right.
For a tree [3,9,20,null,null,15,7], the output is [[9],[3,15],[20],[7]].
Constraints: , node values from to .