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

int a[] = {2,1,2,3,4};

int b[] = {0};

for(int i = 0; i < a.size; i++)
{

    if(a[i] == a[i+1])
    {
         //duplicate found
         //copy it to second array
        b[i] = a[i];
    }
}