主要功能是用递归实现输入字符串的反向输出。特点通过子函数返回反向的字符串，而不是在子函数中逐个字符的输出。特别是自己定义了几个实现字符串操作的子函数。-Main function is to use recursive reverse output of the input string. Features reverse through the subroutine to return a string, rather than the output subroutine in the cha