模拟linux下的shell中的目录系统，把目录抽象为一个多叉树，目录的打印比较有创意，虽然和pstree的打印结果很像，程序中用了大量的递归，简化了代码的长度，增强了可读性-Simulation of the shell under linux directory in the system, the abstract for more than one directory-tree, print directory of more creative, though, and print th