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Find duplicates in an array

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Given an array a of size N which contains elements from 0 to N-1, you need to find all the elements occurring more than once in the given array. Return the answer in ascending order. If no such element is found, return list containing [-1].

Note: The extra space is only for the array to be returned. Try and perform all operations within the provided array.

Example 1:

Input:
N = 4
a[] = {0,3,1,2}
Output:
-1
Explanation:
There is no repeating element in the array. Therefore output is -1.
Example 2:

Input:
N = 5
a[] = {2,3,1,2,3}
Output:
2 3
Explanation:
2 and 3 occur more than once in the given array.m
Your Task:
Complete the function duplicates() which takes array a[] and n as input as parameters and returns a list of elements that occur more than once in the given array in a sorted manner.

Expected Time Complexity: O(n).
Expected Auxiliary Space: O(n).