# Lecture 10 --- Iterator Implementation & Linked Lists

- Iterator Implementation (in Vectors)
- Building our own basic linked lists:  
  – Stepping through a list  
  – Basic operations

## 10.1 Review: Iterators and Iterator Operations

What is the output of this program?

```cpp
#include <list>
#include <iostream>

int main(){

	std::list<int> lst;
	lst.push_back(150);
	lst.push_back(250);
	lst.push_back(350);
	lst.push_back(450);
	
	std::list<int>::iterator itr;
	itr = lst.begin();
	++itr;
	*itr += 5;

	std::list<int>::iterator itr2 = lst.begin();
	while(itr2 != lst.end()){
		std::cout << *itr2 << std::endl;
		itr2++;
	}
	
}
```

## 10.2 Iterator Implementation

Here we extend our implemention of vector so as to include iterators. Review the written code here: [vec.h](vec.h) and [vec_main.cpp](vec_main.cpp).

## 10.3 Working towards our own version of the STL list

- Our discussion of how the STL list&lt;T&gt; is implemented has been intuitive: it is a “chain” of objects.
- Now we will study the underlying mechanism — linked lists.
- This will allow us to build custom classes that mimic the STL list class, and add extensions and new features.

## 10.4 Objects with Pointers, Linking Objects Together

- The two fundamental mechanisms of linked lists are:  
  – creating objects with pointers as one of the member variables, and  
  – making these pointers point to other objects of the same type.  
- These mechanisms are illustrated in the following program:

```cpp
template <class T>
class Node {
public:
	T value;
	Node* ptr;
};
int main() {
	Node<int>* ll; // ll is a pointer to a (non-existent) Node
	ll = new Node<int>; // Create a Node and assign its memory address to ll
	ll->value = 6; // This is the same as (*ll).value = 6;
	ll->ptr = NULL; // NULL == 0, which indicates a "null" pointer
	Node<int>* q = new Node<int>;
	q->value = 8;
	q->ptr = NULL;
	// set ll's ptr member variable to
	// point to the same thing as variable q
	ll->ptr = q;
	cout << "1st value: " << ll->value << "\n"
	<< "2nd value: " << ll->ptr->value << endl;
}
```

![alt text](linking.png "linking")

- Play this [animation](https://jidongxiao.github.io/CSCI1200-DataStructures/animations/lists/creation/index.html) to see how this program works.

## 10.5 Definition: A Linked List

- The definition is recursive: A linked list is either:
  - Empty, or
  - Contains a node storing a value and a pointer to a linked list.
- The first node in the linked list is called the head node and the pointer to this node is called the head pointer.
The pointer’s value will be stored in a variable called head.

## 10.6 Visualizing Linked Lists

![alt text](linked_list.png "linked list")

- The head pointer variable is drawn with its own box. It is an individual variable. It is important to have a
separate pointer to the first node, since the “first” node may change.
- The objects (nodes) that have been dynamically allocated and stored in the linked lists are shown as boxes,
with arrows drawn to represent pointers.
  - Note that this is a conceptual view only. The memory locations could be anywhere, and the actual values
of the memory addresses aren’t usually meaningful.
- The last node MUST have NULL for its pointer value — you will have all sorts of trouble if you don’t ensure this!
- You should make a habit of drawing pictures of linked lists to figure out how to do the operations.

## 10.7 Basic Mechanisms: Stepping Through the List

- We’d like to write a function to determine if a particular value, stored in x, is also in the list.
- We can access the entire contents of the list, one step at a time, by starting just from the head pointer.
  – We will need a separate, local pointer variable to point to nodes in the list as we access them.
  – We will need a loop to step through the linked list (using the pointer variable) and a check on each value

## 10.8 Exercise: Write is_there

```cpp
template <class T> bool is_there(Node<T>* head, const T& x) {








}
```

- If the input linked list chain contains n elements, what is the order notation of is_there?

## 10.9 Overview: Adding an Element at the Front of the List

- Goal: place a new node at the beginning of the list.
- We must create a new node.
- We must permanently update the head pointer variable's value.
  Therefore, we must pass the pointer variable **by reference.**

## 10.10 Exercise: Write push_front

```cpp
template <class T> void push_front( Node<T>* & head, T const& value ) {






}
```

- Play this [animation](https://jidongxiao.github.io/CSCI1200-DataStructures/animations/lists/push_front/index.html) to see how push_front works.

- If the input linked list chain contains n elements, what is the order notation of the implementation of push_front?

## 10.11 Basic Mechanisms: Pushing on the Back

- Goal: place a new node at the end of the list.
- We must step to the end of the linked list, remembering the pointer to the last node.
  - This is an O(n) operation and is a major drawback to the ordinary linked-list data structure we are
discussing now. We will correct this drawback by creating a slightly more complicated linking structure
in our next lecture.
- We must create a new node and attach it to the end.
- We must remember to update the head pointer variable’s value if the linked list is initially empty.
  – Hence, in writing the function, we must pass the pointer variable** by reference.**
  
## 10.12 Exercise: Write push_back

```cpp
template <class T> void push_back( Node<T>* & head, T const& value ) {






}
```

- Play this [animation](https://jidongxiao.github.io/CSCI1200-DataStructures/animations/lists/push_back/index.html) to see how push_back works.

- If the input linked list chain contains n elements, what is the order notation of this implementation of push_back?

## 10.13 Inserting a Node into a Singly-Linked List

- With a singly-linked list, we’ll need a pointer to the node before the spot where we wish to insert the new
item.
- If p is a pointer to this node, and x holds the value to be inserted, then the following code will do the insertion.
Draw a picture to illustrate what is happening.

```cpp
Node<T> * q = new Node<T>; // create a new node
q -> value = x; // store x in this node
q -> next = p -> next; // make its successor be the current successor of p
p -> next = q; // make p's successor be this new node
```

- Play this [animation](https://jidongxiao.github.io/CSCI1200-DataStructures/animations/lists/insert/index.html) to see how this code snippet works.

- Note: This code will not work if you want to insert x in a new node at the front of the linked list. Why not?

## 10.14 Removing a Node from a Singly-Linked List

- The remove operation itself requires a pointer to the node before the node to be removed.
- Suppose p points to a node that should be removed from a linked list, q points to the node before p, and head
points to the first node in the linked list. Note: Removing the first node is an important special case.
- Write code to remove p, making sure that if p points to the first node that head points to what was the second
node and now is the first after p is removed. Draw a picture of each scenario.

## 10.15 Exercise: Singly-Linked List Remove All

Write a recursive function to delete all nodes in a linked list. Here’s the function prototype:

```cpp
template <class T> void RemoveAll(Node<T>*& head) {






}
```

## 10.16 Exercise: Singly-Linked List Copy

Write a recursive function to copy all nodes in a linked list to form an new linked list of nodes with identical structure
and values. Here’s the function prototype:

```cpp
template <class T> void CopyAll(Node<T>* old_head, Node<T>*& new_head) {





}
```


<!-- ## 10.16 Leetcode Exercises

- [Leetcode problem 27: Remove Element](https://leetcode.com/problems/remove-element/). Solution: [p27_removeelement.cpp](../../leetcode/p27_removeelement.cpp)
- [Leetcode problem 58: Length of Last Word](https://leetcode.com/problems/length-of-last-word/). Solution: [p58_lengthoflastword.cpp](../../leetcode/p58_lengthoflastword.cpp)
- [Leetcode problem 283: Move Zeroes](https://leetcode.com/problems/move-zeroes/). Solution: [p283_movezeroes.cpp](../../leetcode/p283_movezeroes.cpp)
-->