DepartmentofComputerScienceandTechnology;
Xi'anJiaotongUniversity;
Xi'an710049;
P.R.China;
DivisionofEngineeringandAppliedSciences;
HarvardUniversity;
Cambridge;
MA02138;
U.S.A.;
DepartmentofComputerScienceandTechnology;
Xi'anJiaotongUniversity;
Xi'an710049;
P.R.China;