Recognize Digit-Square Sequences That Reach One
Company: Google
Role: Software Engineer
Category: Coding & Algorithms
Difficulty: easy
Interview Round: Other
### Problem
Starting from positive n, repeatedly replace the number by the sum of the squares of its decimal digits. A happy number eventually reaches 1; otherwise the process repeats in a cycle not containing 1. Return whether n is happy.
```cpp
bool isHappy(int n);
```
### Examples
```text
Input: n = 7
Output: true
```
Its sequence is 7→49→97→130→10→1.
```text
Input: n = 4
Output: false
```
### Constraints
- `1 <= n <= 2147483647`.
Overview: Recognize happy numbers by exact decimal digit-square transitions and finite-state repetition detection.