Happy Number - Two Pointers / Advanced

[algo.monster]

https://algo.monster/problems/happy_number

[Flavio]

The example 2 is wrong.
4^2 + 2^2 is not 18, it is 20.

[am1]

Thank you for pointing out. We’ve fixed it.

[mav]

Can someone explain how the time complexity is O(n) ?