The solution to the problem with first n natural nos. can easily be solved by XOR-ing all the elements, but to my problem I couldn't think of any such approach. So I suggested of using a dictionary(python) or a map(c++) and store the count of each variable. He asked me of some more general approach that is without using dict or map. So the only option was to make an array of size of maximum possible element in the given array an store the count.