Upgrade: sorted link list

    New feature: compare function now available

        When alloc a new sorted link list,
        you should give it a compare function to decide the bigger element.
New file: linkstack.h

    Implementation of link stack, using the node.h as the data node.

    New function: pop, push, peek, isempty.
Modified: makefile

    Added: linkstack.h to compile
New file: tools.h

    move all common tools into a individual file

Modified: makefile

    add the 'tools.h' into make target

Modified: main.cpp

    you see