"""
1426. Counting Elements
Difficulty: Easy
https://leetcode.com/problems/counting-elements/

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Given an integer array arr, count how many elements x there are, such
that x + 1 is also in arr. If there are duplicates in arr, count them
separately.



Example 1:

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

Example 2:

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.



Constraints:

        • 1 <= arr.length <= 1000

        • 0 <= arr[i] <= 1000
"""


class Solution:
    def countElements(self, arr: List[int]) -> int:
        arr_set = set(arr)
        count = 0
        for n in arr:
            sum = n + 1
            if sum in arr_set:
                count += 1
        return count