Counting Elements Python Solution with Tests

lcpy gen -n 1426   # by problem number
lcpy gen -s counting_elements   # by problem name

Input: arr = [1,2,3]
Output: 2
Explanation: 1 and 2 are counted cause 2 and 3 are in arr.

Input: arr = [1,1,3,3,5,5,7,7]
Output: 0
Explanation: No numbers are counted, cause there is no 2, 4, 6, or 8 in arr.

class Solution:
    # Time: O(n)
    # Space: O(n)
    def count_elements(self, arr: list[int]) -> int:
        counts: set[int] = set(arr)
        return sum(x + 1 in counts for x in arr)