PayPal Software Intern interview questions

I had met an eBay/Paypal rep at my university's career fair and the person happened to be working on the same thing that I was interested in - Big Data. I was contacted by the HR almost a month later and a phone interview was quickly set up. The phone interview was fairly easy with questions being asked about my resume and projects, Hadoop, MapReduce and some very basic and standard Java and SQL questions. I was confident of clearing it and at least getting called on site. A week later, I was invited to come on site and a date was set up. I was to interview with 3 people back to back over a duration of 2 and a half hours. The first interview was mostly coding. I was asked to reverse a number, then to implement the insert and delete functions of a priority queue using any data structure of my choice. I answered wrongly, but was told that a tree map would do the job. The last question in that round was how to delete a number from a linked list. In the second one, I was asked a few basic SQL statements, followed by a question on normalization - what it is and how its useful. And then I was asked to elaborate on the differences between OLAP and OLTP. The third interview was with the same person who had interviewed me over phone. She asked me mostly technical, but non-coding questions like - what is the general architecture of a web application, name a few design patterns and explain what a singleton is, and some optimization question involving databases. Overall, it was an easy interview, but I was under-prepared. Although I hoped to get in, I would not have deserved it if I actually had.