Department of Computer Science and Information Engineering, National Taiwan University, Taipei, Taiwan. ythuang@acb.csie.ntu.edu.tw|Department of Computer Science and Information Engineering, National Taiwan University, Taipei, Taiwan. ythuang@acb.csie.ntu.edu.tw|Department of Computer Science and Information Engineering, National Taiwan University, Taipei, Taiwan. ythuang@acb.csie.ntu.edu.tw|Department of Computer Science and Information Engineering, National Taiwan University, Taipei, Taiwan. ythuang@acb.csie.ntu.edu.tw