n-th term of the rise & reset sequence
Haskell, 27 26 bytes



([z|k&lt;-[1..],z&lt;-[1..k]]!!)


Try it online!

Thanks @DanD. for -1 byte!

This is an anonymous function, creating the infinite sequence an just returning the n-th element thereof: [[1..k]| k&lt;-[1..]] produces an infinite list of list: [[1],[1,2],[1,2,3],[1,2,3,4],...]. To concatenate these we can write [z|k&lt;-[1..],z&lt;-[1..k]] which results in [1,1,2,1,2,3,1,2,3,4,...] and finally (...!!) accepts the input n (pointless notation) and returns the n-th term (0-based).