这是一道大学生信息学奥林匹克竞赛的试题的解答，里面应用了特殊的控制手段来限制递归算法的递归深度，这种手段在利用递归结决问题时非常有借鉴意义。-students Informatics Olympiad answer the questions, inside a special application of the control measures to limit the recursive algorithm recursive depth, this means the use of recursive guitar at the very outstanding issues, he said.