I. INTRODUCTION  
II. BACKGROUND  
A. Supervised Learning  
B. Markov Decision Process (MDP)  
C. Reinforcement Learning Algorithms  
D. Multi-agent Reinforcement Learning  
III. TAXONOMY OF LEARNING METHODS FOR ROUTING PROBLEMS  
IV. HYBRID METHODS  
V. SINGLE VS. MULTIPLE VRP FORMULATIONS  
VI. THE FUTURE RESEARCH DIRECTIONS