Hey! I've got a good one for you today!

Given an array of integers, 1 ≤ a[i] ≤ n (n = size of array), some elements appear twice and others appear once.

Find all the elements that appear twice in this array.

Could you do it without extra space and in O(n) runtime?

    Example:
    Input:
    [4,3,2,7,8,2,3,1]

    Output:
    [2,3]

See if you can solve it before heading to the answer!