1. What is a postfix expression?

Postfix expression is an expression where each operator follows its operands. User does not have to group the sub-expressions in parenthesis or consider operator precedence.

2. What is data abstraction?

A powerful tool that breaks down complex data problems into manageable chunks by specifying the data objects involved and the operations without worrying about how the data will be stored and represented.

3. Differences between an ARRAY and a STACK?

STACK follows the LIFO concept where data is accessed following a sequence where the last data stored is the first out retrieved.

ARRAY does not follow any particular concept or pattern or order instead it can be accessed by referring to the indexed element within the array.

4. What is an AVL tree?

AVL tree is a type of a binary tree that is always on a state of partially balanced state. The balance gets to be measured as the difference between the heights of the subtrees from the root.

5. What is a doubly-linked list?

A type of linked list where the transversal of data elements can be done from both directions. The list has two links in every node, one linking to the next node and another one connects to the previous node.

6. What is Huffman’s algorithm?

An algorithm that is used to create an extended binary tree with minimum weighted path lengths from the provided weights. It uses a table that contains the frequency of the data element occurrences.

7. How does one search for a target key in a linked list?

You can search for a target key in a linked list using the sequential search. Each node is transversed and compared with the target key and if it is different, then it follows the link to the next node. The traversal continues until the target key is found or if the last node is reached.

8. What is an Infix notation?

An operator that is written in between the operands.

```Example A*(B+C)/D
```

9. What is the Prefix notation sometimes also referred to as the polish notation?

Operators that are written before the operands. The expression can be written as;

```/*A+BCD
```

10. What is a circular linked list?

A linked list where all the nodes are connected to form a circle. There is no NULL at the end and can easily be a singly circular linked list or a doubly circular linked list.

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Quote: The moon looks upon many night flowers; the night flowers see but one moon. – Jean Ingelow