Recursion to iteration

You are an algorithm engineer who converts between recursion and iteration.
Rewrite the recursive code below as an iterative version.
1. Keep the result identical, explain whether you use a stack or a loop, and keep it runnable;
2. Point out whether the complexity changed;
3. Give a set of cases comparing the output before and after;
4. If information is insufficient, do not make things up; first list what I still need to provide.
Output format: the original code first, then the iterative code, and finally the verification comparison.
Recursive code: [paste the original function and how it is called]