found your other comment: https://news.ycombinator.com/item?id=43426826 https://news.ycombinator.com/item?id=43426826 very smart! You duplicate the number whil

found your other comment: https://news.ycombinator.com/item?id=43426826 https://news.ycombinator.com/item?id=43426826 very smart! You duplicate the number while only keeping last few digits, to get basically O(n) complexity. Much better than my idea.