1. How does indexing work?

2. What are the SOLID principles?

3. Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid. An input string is valid if:\n\n1. Open brackets must be closed by the same type of brackets.\n2. Open brackets must be closed in the correct order.\n\nReturn true if the string is valid; otherwise, return false.

4. What is the process of authentication and how does JSON Web Token (JWT) work?