I actually done the problem iteratively and already forget about recursion which make me a bit more time imagining during conversion process (I just need to change and add 1 variable) honestly I should ask for more time to complete the task. the interviewer asking me to optimized the code though in which in my opinion should prob just ask me do in recursion than saying to optimized is since recursion is mostly less efficient than iterative code actually basic recursion so if you study most basic recursion it should be easy. for me though I kinda have problem converting it in 10 minutes due unpreparedness prob would done it in 12 - 15 min though .