Department of Computer Science and Information Engineering, National Taiwan University, Taipei 106, Taiwan, ROC. cychen@mars.csie.ntu.edu.tw|Department of Computer Science and Information Engineering, National Taiwan University, Taipei 106, Taiwan, ROC. cychen@mars.csie.ntu.edu.tw|Department of Computer Science and Information Engineering, National Taiwan University, Taipei 106, Taiwan, ROC. cychen@mars.csie.ntu.edu.tw