I. Introduction
Swarm robotics has achieved significant progress benefiting from the development of artificial intelligent [1]. Swarm robotics can be used in many applications, especially those require large amount of robots and time as well as difficult or dangerous for human beings, e.g. foraging [2], surveillance [3], monitoring [4] and search-and-rescue [5]. These applications can be abstracted as a multiple target searching problem with restrictions in the environment, such as obstacles. Searching strategies for solving this abstracted problem can be adapted to many applications and remains an important task for swarm robotics researchers.