The unordered configuration space of n points on a graph Γ, denoted here by UC n (Γ), can be viewed as the space of all configurations of n unlabeled robots on a system of one-dimensional tracks, which is interpreted as a graph Γ. The topology of these spaces is related to the number of vertices of degree greater than 2; this number is denoted by m(Γ). We discuss a combinatorial approach to compute the topological complexity of a "discretized" version of this space, UD n (Γ), and give results for certain classes of graphs. In the first case, we show that for a large class of graphs, as long as the number of robots is at least twice the number of essential vertices, then TC(UD n (Γ)) = 2m(Γ) + 1. In the second, we show that as long as the number of robots is at most half the number of vertex-disjoint cycles in Γ, we have TC(UD n (Γ)) = 2n + 1.