说明：  读懂C++递归程序pdf很清淅的格式，贺利坚制作的“读懂递归”电子书。在程序设计中，数据描述和算法表达也常用递归，通常把一个大型复杂的问题层层转化为一个与原问题相似的规模较小的问题来求解，递归策略只需少量的程序就可描述出解题过程所需要的多次重复计算，大大地减少了程序的代码量。
(Understand the C ++ recursive program pdf very clear format, "read and understand recursive" e-book produced by He Lijian. In program design, data description and algorithm expression are also commonly used recursively. Usually, a large and complex problem is transformed into a small-scale problem similar to the original problem to be solved. The recursive strategy can be described by a small number of programs. The repeated calculations required during the problem-solving process greatly reduce the amount of code in the program.)