Find duplicate element in array in time O(n)

Question

I have been asked this question in a job interview and I have been wondering about the right answer.

You have an array of numbers from 0 to n-1, one o

Answer 1

You could proceed as follows:

1. sort your array by using a Linear-time sorting algorithm (e.g. Counting sort) - O(N)
2. scan the sorted array and stop as soon as two consecutive elements are equal - O(N)