137. binary tree vertical traversal

Traverse a binary tree vertically from left to right, from top to bottom, the binary tree contains positive integers as node values.

For above binary tree, vertical traversal should return

[6,4,2,7,1,9,10,3,8,5]

If two nodes are at the same position, their order should inherit from their parent node. For example, 9 and 10 are at the same position, since 7 is before 8, so 9 should be before 10.