1. Array reverse, Expression tree and Binary tree. Simple In-Memory cache. 2. Merge 2 sorted arrays into a single one. Find duplicates no. in an array of natural no. 3. Function to return true if array contains three elements whose sum is X 4. When do you use HashMap vs TreeMap vs LinkedHashMap? 5. ArrayList vs LinkedList. How HashMap works internally. 6. Reverse a stack using just push and pop no extra space. 7. Design LLD for Instagram/ photo-sharing app, Design A Cron Job scheduler. 8. Design HLD for Twitter system/ automated parking lot to minimize the entry and exit times.