A: When he was a fresh PhD student, his final project in AI course is to search for a target using a P3DX robot and Kinect sensor. He took 1 month to finish the perception algorithms (e.g, SLAM, detection and location estimation of the target). However, the decision-making algorithm, finding an optimal search path, is too difficult for him to be solved. After reading 70+ papers, he found that it includes 3 NP-hard problems.