Cvent Interview Question

First they asked simple OOP questions followed by 2 java programs I would say they are easy level. 1. First program is verify if a given number is prime. I did it and tried to enhance it for optimal solution. Interviewer was looking for specific optimal condition which I didnt provided. 2. Second program is given an integer array and a sum find 2 numbers in array whose sum is equal to given number. Again looking for optimal solution, interviewer given tip to use a hash set .