Problem sets
UVA Online Judge https://onlinejudge.org
USACO Training Program Gateway https://train.usaco.org
PKU Judge Online http://poj.org
Problem Classification
Method to Solve https://cpbook.net/methodstosolve from https://www.comp.nus.edu.sg/~stevenha/index.html
The Algorithmist https://algorithmist.com/wiki/Main_Page
Lucky 貓的 UVA 園地(原站連不上) http://www3.tcgs.tc.edu.tw/~sagit/luckycat/
Reference: 《大學程式能力檢定 CPE 秘笈》