Merge Folder Trees

Two folder trees are given as lists a and b of [name, parent] entries in preorder. Entry i is a folder called name inside the folder at index parent, and the root has parent -1. Folders inside one folder have distinct names.

If the roots have different names, return []. Otherwise merge b into a: folders with the same name inside the same merged folder become one, recursively. Inside each merged folder, the folders from a come first in their original order, then the folders only in b in their original order. Return the merged tree in the same encoding.