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Finding distinct values from any collection has to take O(n) at least. In this case O(n*m) where n is total # nodes in outer list where each node contains linked list of m nodes.
SO traversing all element once is required in any case.
Now when you traverse them put into hash data structure with key as node value and value as count. Once you traverse entire linked list(nm times) then just query hash data structure with value=1 and get the 7th one..
SO it will be O(n*m) to traverse linked list and O(n*m) to put into hash and constant time operation to query hash structure.
So it will be done in O(n*m) time...

What is most unique.? It's like saying coldest ice...:-)..

What do you mean by "You have a linked list in which each node is another linked list." ?
What does a Node structure look like?

what does it mean by "7th most unique element" ?

Iterate through all the nodes and put the node value as key and value as no. of occurance of the num.. Once done , iterate through hash map and sort it based on value get the 7th number for ascending order and key corresponding to it is 7th most unique element

Create a hashMap key = num and value= count. Create a MinHeap and 7th operation of heapification will ive required value.

Build a min-heap which has 7 nodes, traverse all nodes and keep on adjusting the min-heap. Finally, the min-heap top is the answer.

Kis tarah ka behuda swal he??