Hangzhou Dianzi University Online Judge (abbr. HDU) is the online judge of Hangzhou Dianzi University. It is getting more and more popular these days that it even overtook POJ. It keeps a lot of problems. Many competitions are held on it. The most famous one is BestCoder. There is no doubt that it is one of the most important online judge for students who attend NOIp or ACM/ICPC. You can easily find a solution report to HDU problems. It is also a good choice for advanced training.