Department of Computer Science, National Tsing Hua University, Hsinchu 30013, Taiwan. cllu@cs.nthu.edu.tw|Department of Computer Science, National Tsing Hua University, Hsinchu 30013, Taiwan. cllu@cs.nthu.edu.tw|Department of Computer Science, National Tsing Hua University, Hsinchu 30013, Taiwan. cllu@cs.nthu.edu.tw