Unique number of occurrences

We will count the number of occurrences of each number in the array. We will then check if the number of occurrences is unique using a hash set.

class Solution:
    def uniqueOccurrences(self, arr: List[int]) -> bool:
        count = collections.Counter(arr)
        return len(count.values()) == len(set(count.values()))

Time complexity: O(n), n is the length of the array
Space complexity: O(n)