Oxigen Software Developer interview questions

first round :Online coding test on Cocubes.com Second Round:(F2F interview) Third Round:(With the CTO) He asked oops concepts output related questions on virtual functions what are virtual constructors what are B trees can you code for that what are avl trees?why they are used? He said he will ask me to write code for avl tree later but at the end he forgot :p what are design patterns? what are singleton classes? is there any keyword to define a singleton class in c

There were 3 rounds. 1. First round was coding round: It was a online coding test on cocubes.They asked 2 coding question. 2. second was technical interview: I was asked about conceptual question of operating system,DBMS,JAVA, C and C++.There was one coding question also. 3. HR round: They asked about joining date and salary confirmation etc.