Description:
From two arguments which are the traversing direction indications and label sequence, both for preorder, construct the corresponding process tree. Each process will print a line which should contains its pid, ppid and the label. The output should be in postorder.
Deadline: 2010 Apr 1 00:05am
Put your file(s) under ~/usp-982/exer2
Exercise 1 of usp-962 is similar. You can check it if you do not have any idea.

Examples :

$ echo $$ 
2419     <------- pid of the shell
$

For the tree,   shell
                  |
                  a
the preorder traversing direction is "du". 
That is: shell "d"own to a,  a "u"p to shell.
$ ./a.out du a  
I'm a, my pid=2757, and my ppid=2419
$

for the tree,   shell
                  | 
                  a
                  | 
                  b
the preorder traversing direction is "dduu". 
That is:  shell "d" to a, a "d" to b, b "u" to a, a "u" to shell
$ ./a.out dduu ab  
I'm b, my pid=2759, and my ppid=2758
I'm a, my pid=2758, and my ppid=2419
$

                shell
                  | 
                  a
                 / \ 
                b   c
$ ./a.out dduduu abc
I'm b, my pid=2761, and my ppid=2760
I'm c, my pid=2762, and my ppid=2760
I'm a, my pid=2760, and my ppid=2419
$

                shell
                  | 
                  a
                 / \ 
                b   d
                |
                c
$ ./a.out ddduuduu abcd
I'm c, my pid=2765, and my ppid=2764
I'm b, my pid=2764, and my ppid=2763
I'm d, my pid=2766, and my ppid=2763
I'm a, my pid=2763, and my ppid=2419

                  shell
                    | 
                    a
                  / | \
                 b  d  e
                / 
               c 
$ ./a.out ddduududuu abcde
I'm c, my pid=2770, and my ppid=2769
I'm b, my pid=2769, and my ppid=2768
I'm d, my pid=2771, and my ppid=2768
I'm e, my pid=2772, and my ppid=2768
I'm a, my pid=2768, and my ppid=2419
$