[vietdoor]

Tham khảo:
http://my.opera.com/Tuanbido/blog/20...me-programming
http://vi.wikipedia.org/wiki/L%E1%BA...1c_h%E1%BA%A1n
http://www.codeprovn.com/forums/view...php?f=32&t=588
http://docs.google.com/View?docid=ddp9sqjm_188f7dbkwvh (Chương 21. Purity (Tinh khiết/Nguyên chất))