Whats N, size of the given array? What if there are elements in the array taht are larger than N? A better strategy would be to simply use a hashmap to put the elements and their counts. everytime you see the number you increment the count. this runs in order n space and time.

Just add all the numbers to a Set and return the sum of all numbers from the Set. Set automatically de-duplicates the numbers.

create another array initialized to N with 0 in each slot. Go through the first array, each time you see a number, add that number to the respective index in the second array. Then after, just count the number of elements that aren't 0. This runs in O(n).