Round 1: Machine Coding

Given list of folders, print the path of a given folder from root. If there is no path to the root folder then return an empty string. Root level folders will have 0 as id. The structure of Folder is like this:
class Folder {
int id;
List subfolders;
String name;
}

Ex:
list = [Folder(0, [7, 3], “abc”), Folder(0, [], “xyz”), Folder(3, [], “pqr”), Folder(8, [], “def”), Folder(7, [9], “ijk), Folder(9, [], “lmn”)]

/abc/ijk/lmn
/abc/pqr
/xyz
?? def/

printPath(9) = “abc” -> “ijk” -> “lmn” printPath(8) = ""
Clarification: There can be multiple root level folders. All other folders have unique ids.
Note: printPath method may be called multiple times concurrently. Design your solution taking that into account
Concurrency in input and printPath()

addFolder(Folder f)
printPath(int index)

Round 2 (DSA): Given an n-ary tree, print the boundary nodes. Was doable using BFS.
Given a list of string which can contain any characters, write functions to serialize it to a file and then desiarlize the file back into the list.

Round 3 (SD): Design whatsapp.

Round 4 (HM): Discussions on past projects and their designs. Challenges and questions on opeational excellence. Some behavioural questions.

Round 5 (BR): Discussions on previous projects. Effeciency and operational excellance.

Rejected as the recruiter told me that I had all positives except the machine coding one. Can someone help me with the machine coding question.