https://medium.com/basecs/a-gentle-introduction-to-graph-theory-77969829ead8
In mathematics, graph theory is the study of graphs, which are mathematical structures used to model pairwise relations between objects. A graph in this context is made up of vertices (also called nodes or points) which are connected by edges (also called links or lines).
The Seven Bridges of Königsberg is a historically notable problem in mathematics. Its negative resolution by Leonhard Euler in 1736 laid the foundations of graph theory and prefigured the idea of topology. ... The problem was to devise a walk through the city that would cross each of those bridges once and only once. https://en.m.wikipedia.org/wiki/Seven_Bridges_of_K%C3%B6nigsberg
The Travelling Salesman Problem describes a salesman who must travel between N cities. The order in which he does so is something he does not care about, as long as he visits each once during his trip, and finishes where he was at first. https://en.wikipedia.org/wiki/Travelling_salesman_problem