3 "Makefile" files are provided to compile your project in multiple and multi-level directories. One in current folder and the other two are placed in "obj/" and "src/". We note them as "./Makefile", "src/Makefile", "obj/Makefile" below.
    Your source file directory can be set as below if needed:
    src/
        Makefile
        a/
            Makefile
            src1.cpp
            src2.cpp
            b/
                Makefile
                src3.cpp
            c/
                Makefile
                src4.cpp
        b/
            Makefile
            d/
                ...
            f/
                ...
            ...
    src-subroutine/
            Makefile
            ...
    ...
    src5.cpp
where the "Makefile"s mentioned above were copies of "src/Makefile". Each "Makefile" will automatically find your "cpp" files-the post fix can be modified in "./Makefile"-in its current directory, and execute "make" in every subdirectory. Recursively, source files in subdirectory will be compiled. Note that source files can also be placed in current directory and you can simply remove folders: "src*".

    "*.o" will be placed in the folder "obj/", and can be linked to an excutable file in "./bin/" if "obj/Makefile" was invoked.
    
    You can customize you compile process by modify "./Makefile", e.g.:
        CC=g++
        SOURCE_POSTFIX=cpp
        ROOT_DIR=$(shell pwd)
        OPTIONS=-O2 -std=c++11 -I$(ROOT_DIR)/include ...
        BIN=a.out
these variables will be export to every Makefile in "src/" and "obj/"

    Some useful target:
        all:OBJ LINK run
        OBJ:$(SUBDIRS) $(CUR_OBJS)
        LINK:OBJ
            make -C obj
        run:LINK
            bin/$(BIN)
        clean:
            -rm -rf $(OBJS_DIR)/*.o; -rm -rf bin/$(BIN)