If you have an input which is having userid and the category of the payments that they made with their credit card given as
UseridList - List<String>
CategoryList - List<List<String>>
find the 2 users with the longest common spending pattern. Return null if none exists.

Eg:

UseridList : {"user1", "user2", "user3", "user4"}
CategoryList: {{"fashion","restaurant","grocery","utility"}, {"fuel","fashion","restaurant","grocery"}, {"fuel","utility","restaurant","grocery"}, {"internet","transportation","water","grocery"}}

Longest repeating sequence is between user1 and user2 which is {"fashion","restaurant","grocery"}.
Output : {"user1","user2"}

Follow up question: Optimal way to get list of users (2 or more) which have the the longest common spending pattern.