## Problems
- [K-Query Online](http://www.spoj.com/problems/KQUERYO/) [^1]

## See also
- [Segment tree]()

## External links
- [Lecture on Merge Sort Tree and Persistence in Data Structures By Sergey Kulik](https://www.youtube.com/watch?v=6x4-lfhHd_Y)

[^1]: Note that the test cases are a little bit shady, sometimes giving queries $i,j$ that don't satisfy $1\leq i\leq j\leq n$. **Blue.Mary** describes the correct behaviour in the comments.