An L(2, 1)-labeling of a graph G is a function f from the vertex set V(G) to the set of all nonnegative integers such that \f(x)-f(y)\ greater than or equal to 2 if d(G)(x,y) =1 and \f(x)-f(y)\ greater than or equal to 1 if d(G)(x,y) = 2. The L(2, 1)-labe
