1. What is the Virtual DOM? Explain in detail.

2. Explain the concept of hoisting in JavaScript.

3. You are given a linked list where the node has two pointers. The first is the regular ‘next’ pointer. The second pointer is called ‘down’ which points to a separate linked list. These down linked lists can also have down linked lists of their own, and so on. The goal is to flatten the linked list into a single-level linked list. Specifically, you need to flatten the list in place, using only constant extra space. The order of nodes in the flattened list should be depth-first.