Peking University JudgeOnline for ACM/ICPC (abbr. POJ) is the online judge of Peking University. It is also opened to public. It is famous for its classic problems. Both the beginners and advancers would like to solve problem on this online judge because it keeps a large number of ACM/ICPC problems and they are good-sorted. You can easily find a lot of plans to improve your programming skills with POJ on the Internet. They're all cut into some small knowledge points like simulation, sort algorithms, graphic theory, number theory, dynamic planning, etc. It is so popular among students (including high school students and university students).
P. S. POJ is such an old online judge that it easily breaks down. The Pascal code of the following problems used to be unable to verify: POJ 1007, POJ 3292, POJ 3984, but now the problems are all fixed and the codes were submitted successfully.