Not really obscure, but I had never heard of it till recently (and most people I've talked had no idea about it): Difference Arrays. Great for when you need to

Not really obscure, but I had never heard of it till recently (and most people I've talked had no idea about it): Difference Arrays. Great for when you need to do ranged updates in constant time. More info: https://codeforces.com/blog/entry/78762 https://codeforces.com/blog/entry/78762