Are two vertices connected in a graph

Problem: How to tell if a path exists between two vertices on a graph?

Answer:

Choose one vertex V as your start; let V’ be the end point. Let R_n and D_n denote sets of of vertices.

Set R_0 = nil, D_0 = {V}, n = 0, found = false;

while D_n .ne. nil begin
	begin if V' in D_n
		found = true
		escape
		end if
	R_n+1 = R_n + D_n
	D_n+1 = neighbours(D_n) - R_n+1
	end