Fix some trivial mistakes.

1. Update README file.
2. Remove the redundant newline in elementary solution.
Enrich the desc and move Pro.18 to elementary.

1. Add the title to the problem (and after the problem number).
2. Move the Pro.18 from easy level to elementary level.
Add more solutions to the easy level problems.

The new added problems' number is: 61, 62, 81, 97, 122, 134, 147, 157.