Given an integer array arr, count how many elements x there
are, such that x + 1 is also in arr.
If there're duplicates in arr, count them seperately.
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's no 2, 4, 6, or 8 in arr.
Example 3:
Input: arr = [1,3,2,3,5,0] Output: 3 Explanation: 0, 1 and 2 are counted cause 1, 2 and 3 are in arr.
Example 4:
Input: arr = [1,1,2,2] Output: 2 Explanation: Two 1s are counted cause 2 is in arr.
Example 5:
Input: arr = [1,1,2] Output: 2 Explanation: Both 1s are counted because 2 is in the array.
Constraints:
1 <= arr.length <= 10000 <= arr[i] <= 1000