* 2020-05-18
** note
   - ex1.py
     - # is commnet.
   - ex2.py
     - I do't find mistakes.
   - ex3.py
     - none
   - ex4.py
     - car_pool_capacity is not defined.
   - ex5.py
     - none
   - ex6.py
     - + is to add two string array.