# Linked list

Jump to: navigation, search

A linked list is a recursive data structure that represents a sequence of elements. It consists of a series of nodes. Each node contains a piece of data, as well as a pointer to the next node. The last element in the list is the empty linked list. The piece of data is called the "first" field of that linked list, and the pointer to the next node is called the "rest" field.

## Types

In a straight-forward linked list, a node's first field contains a value (string, number, etc.), while the second field will contain another linked list. Using this structure, a series of nested linked lists can form a list of values.

A deep linked list is slightly different. The first and second fields contain another linked list. A good way to visualize linked lists is to draw them out.

## Functional ADT

The ADT of a linked list is independent of its implementation. The functions are:

• `link(elem, list)` – returns a linked list with `elem` as the first item and `list` as the rest of the list
• `first(list)` – returns the first field of linked list `list`
• `rest(list)` – returns the rest field of linked list `list`
• `empty` – the empty linked list

The following are implementations of the ADT:

• with tuples:
```empty = lambda: 42

def link(element, list=empty):
return (element, list)

def first(list):
return list[0]

def second(list):
return list[1]```
• with `cons`:
```empty = lambda: 42

def link(element, list=empty):
return cons(element, list)

def first(list):
return car(list)

def rest(list):
return cdr(list)```

## OOP ADT

The OOP ADT is:

• `LinkedList(elem, list)` – returns a linked list with `elem` as the first item and `list` as the rest of the list
• `list.first` – returns the first field of linked list `list`
• `list.rest` – returns the rest field of linked list `list`
• `LinkedList.empty` – the empty linked list