> ## 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.

# Happy Number Python Solution with Tests

> Tested Python solution for LeetCode 202 with 14 pytest cases. Generate a practice environment with lcpy.

LeetCode 202, Easy. Topics: Hash Table, Math, Two Pointers. [View on LeetCode](https://leetcode.com/problems/happy-number/description/).

Generate this problem as a practice environment: tested reference solution, 14 [parametrized pytest cases](/practice/testing), and a playground notebook:

```bash theme={"theme":{"light":"github-light","dark":"github-dark"}}
lcpy gen -n 202   # by problem number
lcpy gen -s happy_number   # by problem name
```

## Problem

Write an algorithm to determine if a number `n` is happy.

A **happy number** is a number defined by the following process:

* Starting with any positive integer, replace the number by the sum of the squares of its digits.
* Repeat the process until the number equals 1 (where it will stay), or it **loops endlessly in a cycle** which does not include 1.
* Those numbers for which this process **ends in 1** are happy.

Return `true` *if* `n` *is a happy number, and* `false` *if not*.

### Examples

```
Input: n = 19
Output: true
```

**Explanation:**
1^2 + 9^2 = 82
8^2 + 2^2 = 68
6^2 + 8^2 = 100
1^2 + 0^2 + 0^2 = 1

```
Input: n = 2
Output: false
```

### Constraints

* 1 \<= n \<= 2^31 - 1

## Solution

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

```python theme={"theme":{"light":"github-light","dark":"github-dark"}}
class Solution:
    # Time: O(log n)
    # Space: O(log n)
    def is_happy(self, n: int) -> bool:
        def digit_square_sum(num: int) -> int:
            total = 0
            while num:
                digit = num % 10
                total += digit * digit
                num //= 10
            return total

        slow, fast = n, digit_square_sum(n)
        while fast != 1 and slow != fast:
            slow = digit_square_sum(slow)
            fast = digit_square_sum(digit_square_sum(fast))
        return fast == 1
```

## Complexity

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

## Tags

[NeetCode 150](/catalog/neetcode-150), [NeetCode 250](/catalog/neetcode-250), [NeetCode All](/catalog/neetcode).