## sorting
* **inversions** in a sequence is a pair of elements that are out of order with respect to the ordering relation. a sorting algorithm is a sequence of operations that reduces inversions to zero.
* a **topological sort** of a diretect graph is a way of ordering the list of nodes such that if `(a, b)` is a edge of the graph, then `a` appears before `b`. it does not work if a graph has cycles or is not directed.
