1. General questions on data structures - deletion of nodes from B Trees, mirror image of B Tree.
2. Questions on complexity.
3. Standard puzzles.
4. Object Oriented Programming concepts.
Advice to non-CS people : Try to solve a few puzzles in your preparation, at least the standard ones like the egg drop puzzle. Because non-CS people are not expected to be well-versed with the syntax of programming languages, ability to solve puzzles will justify your grasp on the logic of a problem's solution.
Also, try to think loud while solving a question in front of the interviewer, don't write your solutions as if you're writing your exams. Written and working code is expected, so practice writing code on paper rather than on your machine.
This is generally the make-or-break round as far as Citi goes. Not that the technical round matters any less, but this is the round where you'll be grilled absolutely. Questions in the HR round are generally developed from the answers you give. There will be standard HR questions like "Why Citi?" and the likes, but you have to give well reasoned and honest answers. Because they're going to get to the depth of your answers in the subsequent counter questions. One question I remember being asked was "Why should we not hire you?". There'll be a couple of panelists for this round. They will try to assess whether you have the knack for understanding the business perspective behind the technology, because at the end of the day, technology is just an enabler for them.
This is typically the elimination round, if you have made till here. The panelist is usually a Senior Vice President or a Director who'll probably be heading your department at Citi. In my case, he turned out to be so. During the time we had our interview, ie, in 2015, there was a project at Citi on memory allocators (which I later realised). The interviewer asked a few common questions based on dynamic memory allocators to many of those who had made it till the third round, among other questions which were a mix of both technical and HR. The interviewer was a no-nonsense guy and expected brief and to the point answers.
For Citi, just be thorough with the basic data structures and sorting algorithms. The most difficult coding questions will probably be in the online round itself. In the interviews, it gets easier. Good communication skills can be your X-factor. And reiterating one of my aforesaid points, be honest in the HR interview and you'll nail it. All the best! :)