Let $G$ be a complete graph. Prove that there always exists a way to assign $n(n-1)/2$ directed edges in a way that the graph will be acyclic (it will contain no directed circle). In other words, prove that every complete graph can be acyclic.
To clarify what I mean: Here's an example of one valid assignment for a 4-vertices complete graph:
It is acyclic, it is complete.
The only observation I have made is that the in-degrees for A-B-D-C respectively are 0,1,2,3; while the out-degrees for A-B-D-C respectively are 3,2,1,0, but don't really know whether it helps me to prove this.
Thank you in advance and sorry if some of the terms I have used aren't accurate, I'm not taking this course in English.