Happy Number - Hint

The tricky part is detecting when you're in a cycle. If you see the same number twice, you're looping and will never reach $1$.

Use a set to track all numbers you've seen. Before computing the next sum, check if the current number is already in the set.

To extract digits, use the pattern from earlier: $d = n \bmod 10$, then $n := \lfloor n / 10 \rfloor$. Repeat until $n = 0$.