In computer science diagrams like this are often used to represent the essential information of a problem. Such a diagram is called a graph. The circles in the diagram are called vertices or nodes of the graph.In a graph it is only important whether two vertices are connected or not. Where the vertices are drawn is immaterial. The same graph can often be drawn in different ways, as is illustrated in the explanation section: Both answer A and the last picture in the explanation are correct diagrams for the board and represent the same graph.