  The Cartesian product of P_2 and P_n is called an n-ladder graph for a
positive integer n. We call two paths P_m and P_n together with some edges each
of which joins a vertex on P_m and a vertex on P_n a generalized (m,n)-ladder
graph. In this paper, we completely characterize the planar generalized ladder
graphs and the outerplanar generalized ladder graphs. A functigraph C(P_n ,f)
is a generalized (n,n)-ladder graph. Consequently, our result solves the
problem posed by A. Chen et al. (2011) to characterize planar functigraphs
C(P_n ,f).
