Dictionary Problems

1. Count word frequency.
2. Find the key with the highest value.
3. Invert a dictionary when values are unique.
4. Merge dictionaries by adding common values.
5. Group words by their first character.
6. Find duplicate values in a dictionary.
7. Sort dictionary entries by value.
8. Build a simple phone book.
9. Convert two lists into a dictionary.
10. Find common keys and compare their values.