degre: degree of a vetrex is the total number of edges connecting the vertex
out-degree and in-degree: for directed graph, degree has in-degree and out-degree
loop: edges that connect ot the same vertex
path: starting from vertex u, and ending at vertex v, there is a sequence of v0, ee1, v1, e2, v3, vk, K is hte length of the path. If u eequals to v, then the path is closed; otherwise it is open
Distance: if there exists a shortest parth of vertex u to v, then the legnth of the path is called distance