---
title: "ByteByteGo: Happy Number"
url: https://perrotta.dev/2025/12/bytebytego-happy-number/
last_updated: 2026-01-03
---


[ByteByteGo: Happy Number](https://bytebytego.com/exercises/coding-patterns/fast-and-slow-pointers/happy-number):

```python
def happy_number(n: int) -> bool:
    seen = (set([n]))

    def apply(n):
        ans = 0

        while n > 0:
            ans += (n % 10) ** 2
            n //= 10

        return ans

    assert apply(23) == 13

    while n != 1:
        n = apply(n)

        if n in seen:
            return False
        else:
            seen.add(n)

    return True
```