> ## Documentation Index
> Fetch the complete documentation index at: https://leetcode-py.wisl.dev/llms.txt
> Use this file to discover all available pages before exploring further.

> ## Agent Instructions
> leetcode-py is a Python LeetCode practice environment generator with one CLI: lcpy. It is not a service or platform.
> Each problem is a directory under leetcode/ with README.md, solution.py, test_solution.py, helpers.py, and playground.ipynb. lcpy gen creates them from JSON templates bundled with the package.
> Examples are backed by tests; copy them verbatim.

# Counting Elements Python Solution with Tests

> Tested Python solution for LeetCode 1426 with 26 pytest cases. Generate a practice environment with lcpy.

LeetCode 1426, [Easy](/catalog/easy). Topics: [Array](/catalog/topics/array), [Hash Table](/catalog/topics/hash-table). [View on LeetCode](https://leetcode.com/problems/counting-elements/description/).

Generate this problem as a practice environment: tested reference solution, 26 [parametrized pytest cases](/practice/testing), and a playground notebook:

```bash theme={"theme":{"light":"github-light","dark":"github-dark"}}
lcpy gen -n 1426   # by problem number
lcpy gen -s counting_elements   # by problem name
```

## Problem

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.

### Examples

```
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.
```

### Constraints

* 1 \<= arr.length \<= 1000
* 0 \<= arr\[i] \<= 1000

## Solution

Reference implementation from [solution.py on GitHub](https://github.com/wislertt/leetcode-py/blob/main/leetcode/counting_elements/solution.py), full suite in [test\_solution.py](https://github.com/wislertt/leetcode-py/blob/main/leetcode/counting_elements/test_solution.py):

```python theme={"theme":{"light":"github-light","dark":"github-dark"}}
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)
```

## Complexity

| Time | Space |
| - | - |
| O(n) | O(n) |

## Tags

[NeetCode All](/catalog/neetcode).


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