L ( h , 1, 1)-labeling of outerplanar graphs

An L ( h , 1, 1)-labeling of a graph is an assignment of labels from the set of integers {0, . . . , λ} to the nodes of the graph such that adjacent nodes are assigned integers of at least distance h ≥ 1 apart and all nodes of distance three or less must be assigned different labels. The aim of the L ( h , 1, 1)-labeling problem is to minimize λ, denoted by λ h , 1, 1 and called span of the L ( h , 1, 1)-labeling. As outerplanar graphs have bounded treewidth, the L (1, 1, 1)-labeling problem on outerplanar graphs can be exactly solved in O ( n 3 ), but the multiplicative factor depends on the maximum degree Δ and is too big to be of practical use. In this paper we give a linear time approximation algorithm for computing the more general L ( h , 1, 1)-labeling for outerplanar graphs that is within additive constants of the optimum values.