I’ve started a blog where I write posts regularly on topics which are difficult to understand and have limited resources online. You may follow the blog here . All topics are discussed with problems from spoj, codechef and codeforces. Currently, it has posts on topics like ‘Inclusion-Exclusion principle’ , ‘Square-Root decompostion’, ‘Sieves’ etc. Please feel free to share/comment/ contribute. Any suggestion/constructive criticism is appreciated!.
Edit : Added new post to help readers get started with problem solving using segment trees.