# 1426. Counting Elements

https://leetcode.com/problems/counting-elements

## Description

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'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 <= 1000`
* `0 <= arr[i] <= 1000`

## ac

```java
```
