As for finding K looping paths, Dreyfus algorithm [12] ran in OðKjV j log jV jÞ time given a shortest tree is available. Fox [13] gave an algorithm to run in OðjV j2 þ KjV j log jV jÞ time. Using a concept of path deletion, Martins [14] developed an algorithm with the worst time to be O(K3|V|). Later, this time was improved to be O(K2|A|) by Azevedo et al. [15] who used an efficient implementation. Martins path deletion was generalized by Villeneuve and Desaulniers [16] who developed the solution for the shortest path problem with forbidden paths. The algorithm of Azevedo et al. [15] formed the basis of Yang and Chen [17] who developed a polynomial time algorithm for finding K shortest looping paths in a traffic–light network. Eppstein [18] used an implicit representation of paths to run in O(|A| + |V|log|V| + K|V|) time. On the basis of Eppsteins algorithm, Jime´nez and Marzal [19] proposed a modified version that improved computational performance in practice. For a traffic–light network, Yang and Chen [20] developed a polynomial time algorithm to find K shortest ‘‘unique-arc walks,’’ which means that the path may include repeated nodes but will exclude repeated arcs. Other recent developments related to K shortest paths can be found in [21,22].