Linked List | Set 1 [Introduction]
Like arrays, Linked List is a linear data structure. Unlike arrays, linked list elements are not stored at a contiguous location; the elements are linked using pointers.
Why Linked List?
Arrays can be used to store linear data of similar types, but arrays have the following limitations.
1] The size of the arrays is fixed: So we must know the upper limit on the number of elements in advance. Also, generally, the allocated memory is equal to the upper limit irrespective of the usage.
2] Inserting a new element in an array of elements is expensive because the room has to be created for the new elements and to create room existing elements have to be shifted but in Linked list if we have the head node then we can traverse to any node through it and insert new node at the required position.
For example, in a system, if we maintain a sorted list of IDs in an array id[].
id[] = [1000, 1010, 1050, 2000, 2040].
And if we want to insert a new ID 1005, then to maintain the sorted order, we have to move all the elements after 1000 [excluding 1000].
Deletion is also expensive with arrays until unless some special techniques are used. For example, to delete 1010 in id[], everything after 1010 has to be moved due to this so much work is being done which affects the efficiency of the code.
Advantages over arrays
1] Dynamic size
2] Ease of insertion/deletion
Drawbacks:
1] Random access is not allowed. We have to access elements sequentially starting from the first node[head node]. So we cannot do binary search with linked lists efficiently with its default implementation. Read about it here.
2] Extra memory space for a pointer is required with each element of the list.
3] Not cache friendly. Since array elements are contiguous locations, there is locality of reference which is not there in case of linked lists.
Representation:
A linked list is represented by a pointer to the first node of the linked list. The first node is called the head. If the linked list is empty, then the value of the head points to NULL.
Each node in a list consists of at least two parts:
1] data[we can store integer, strings or any type of data].
2] Pointer [Or Reference] to the next node[connects one node to another]
In C, we can represent a node using structures. Below is an example of a linked list node with integer data.
In Java or C#, LinkedList can be represented as a class and a Node as a separate class. The LinkedList class contains a reference of Node class type.
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// A linked list node
struct Node {
int data;
struct Node* next;
};
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class Node {
public:
int data;
Node* next;
};
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class LinkedList {
Node head; // head of the list
/* Linked list Node*/
class Node {
int data;
Node next;
// Constructor to create a new node
// Next is by default initialized
// as null
Node[int d] { data = d; }
}
}
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# Node class
class Node:
# Function to initialize the node object
def __init__[self, data]:
self.data = data # Assign data
self.next = None # Initialize
# next as null
# Linked List class
class LinkedList:
# Function to initialize the Linked
# List object
def __init__[self]:
self.head = None
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class LinkedList {
// The first node[head] of the linked list
// Will be an object of type Node [null by default]
Node head;
class Node {
int data;
Node next;
// Constructor to create a new node
Node[int d] { data = d; }
}
}
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var head; // head of the list
/* Linked list Node*/
class Node
{
// Constructor to create a new node
// Next is by default initialized
// as null
constructor[val] {
this.data = val;
this.next = null;
}
}
// This code is contributed by gauravrajput1
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First Simple Linked List in C Let us create a simple linked list with 3 nodes.
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// A simple CPP program to introduce
// a linked list
#include
using namespace std;
class Node {
public:
int data;
Node* next;
};
// Program to create a simple linked
// list with 3 nodes
int main[]
{
Node* head = NULL;
Node* second = NULL;
Node* third = NULL;
// allocate 3 nodes in the heap
head = new Node[];
second = new Node[];
third = new Node[];
/* Three blocks have been allocated dynamically.
We have pointers to these three blocks as head,
second and third
head second third
| | |
| | |
+---+-----+ +----+----+ +----+----+
| # | # | | # | # | | # | # |
+---+-----+ +----+----+ +----+----+
# represents any random value.
Data is random because we haven’t assigned
anything yet */
head->data = 1; // assign data in first node
head->next = second; // Link first node with
// the second node
/* data has been assigned to the data part of first
block [block pointed by the head]. And next
pointer of the first block points to second.
So they both are linked.
head second third
| | |
| | |
+---+---+ +----+----+ +-----+----+
| 1 | o----->| # | # | | # | # |
+---+---+ +----+----+ +-----+----+
*/
// assign data to second node
second->data = 2;
// Link second node with the third node
second->next = third;
/* data has been assigned to the data part of the second
block [block pointed by second]. And next
pointer of the second block points to the third
block. So all three blocks are linked.
head second third
| | |
| | |
+---+---+ +---+---+ +----+----+
| 1 | o----->| 2 | o-----> | # | # |
+---+---+ +---+---+ +----+----+ */
third->data = 3; // assign data to third node
third->next = NULL;
/* data has been assigned to the data part of the third
block [block pointed by third]. And next pointer
of the third block is made NULL to indicate
that the linked list is terminated here.
We have the linked list ready.
head
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+---+---+ +---+---+ +----+------+
| 1 | o----->| 2 | o-----> | 3 | NULL |
+---+---+ +---+---+ +----+------+
Note that only the head is sufficient to represent
the whole list. We can traverse the complete
list by following the next pointers. */
return 0;
}
// This code is contributed by rathbhupendra
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// A simple C program to introduce
// a linked list
#include
#include
struct Node {
int data;
struct Node* next;
};
// Program to create a simple linked
// list with 3 nodes
int main[]
{
struct Node* head = NULL;
struct Node* second = NULL;
struct Node* third = NULL;
// allocate 3 nodes in the heap
head = [struct Node*]malloc[sizeof[struct Node]];
second = [struct Node*]malloc[sizeof[struct Node]];
third = [struct Node*]malloc[sizeof[struct Node]];
/* Three blocks have been allocated dynamically.
We have pointers to these three blocks as head,
second and third
head second third
| | |
| | |
+---+-----+ +----+----+ +----+----+
| # | # | | # | # | | # | # |
+---+-----+ +----+----+ +----+----+
# represents any random value.
Data is random because we haven’t assigned
anything yet */
head->data = 1; // assign data in first node
head->next = second; // Link first node with
// the second node
/* data has been assigned to the data part of the first
block [block pointed by the head]. And next
pointer of first block points to second.
So they both are linked.
head second third
| | |
| | |
+---+---+ +----+----+ +-----+----+
| 1 | o----->| # | # | | # | # |
+---+---+ +----+----+ +-----+----+
*/
// assign data to second node
second->data = 2;
// Link second node with the third node
second->next = third;
/* data has been assigned to the data part of the second
block [block pointed by second]. And next
pointer of the second block points to the third
block. So all three blocks are linked.
head second third
| | |
| | |
+---+---+ +---+---+ +----+----+
| 1 | o----->| 2 | o-----> | # | # |
+---+---+ +---+---+ +----+----+ */
third->data = 3; // assign data to third node
third->next = NULL;
/* data has been assigned to data part of third
block [block pointed by third]. And next pointer
of the third block is made NULL to indicate
that the linked list is terminated here.
We have the linked list ready.
head
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+---+---+ +---+---+ +----+------+
| 1 | o----->| 2 | o-----> | 3 | NULL |
+---+---+ +---+---+ +----+------+
Note that only head is sufficient to represent
the whole list. We can traverse the complete
list by following next pointers. */
return 0;
}
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// A simple Java program to introduce a linked list
class LinkedList {
Node head; // head of list
/* Linked list Node. This inner class is made static so that
main[] can access it */
static class Node {
int data;
Node next;
Node[int d]
{
data = d;
next = null;
} // Constructor
}
/* method to create a simple linked list with 3 nodes*/
public static void main[String[] args]
{
/* Start with the empty list. */
LinkedList llist = new LinkedList[];
llist.head = new Node[1];
Node second = new Node[2];
Node third = new Node[3];
/* Three nodes have been allocated dynamically.
We have references to these three blocks as head,
second and third
llist.head second third
| | |
| | |
+----+------+ +----+------+ +----+------+
| 1 | null | | 2 | null | | 3 | null |
+----+------+ +----+------+ +----+------+ */
llist.head.next = second; // Link first node with the second node
/* Now next of the first Node refers to the second. So they
both are linked.
llist.head second third
| | |
| | |
+----+------+ +----+------+ +----+------+
| 1 | o-------->| 2 | null | | 3 | null |
+----+------+ +----+------+ +----+------+ */
second.next = third; // Link second node with the third node
/* Now next of the second Node refers to third. So all three
nodes are linked.
llist.head second third
| | |
| | |
+----+------+ +----+------+ +----+------+
| 1 | o-------->| 2 | o-------->| 3 | null |
+----+------+ +----+------+ +----+------+ */
}
}
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# A simple Python program to introduce a linked list
# Node class
class Node:
# Function to initialise the node object
def __init__[self, data]:
self.data = data # Assign data
self.next = None # Initialize next as null
# Linked List class contains a Node object
class LinkedList:
# Function to initialize head
def __init__[self]:
self.head = None
# Code execution starts here
if __name__=='__main__':
# Start with the empty list
llist = LinkedList[]
llist.head = Node[1]
second = Node[2]
third = Node[3]
'''
Three nodes have been created.
We have references to these three blocks as head,
second and third
llist.head second third
| | |
| | |
+----+------+ +----+------+ +----+------+
| 1 | None | | 2 | None | | 3 | None |
+----+------+ +----+------+ +----+------+
'''
llist.head.next = second; # Link first node with second
'''
Now next of first Node refers to second. So they
both are linked.
llist.head second third
| | |
| | |
+----+------+ +----+------+ +----+------+
| 1 | o-------->| 2 | null | | 3 | null |
+----+------+ +----+------+ +----+------+
'''
second.next = third; # Link second node with the third node
'''
Now next of second Node refers to third. So all three
nodes are linked.
llist.head second third
| | |
| | |
+----+------+ +----+------+ +----+------+
| 1 | o-------->| 2 | o-------->| 3 | null |
+----+------+ +----+------+ +----+------+
'''
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// A simple C# program to introduce a linked list
using System;
public class LinkedList {
Node head; // head of list
/* Linked list Node. This inner class is made static so that
main[] can access it */
public class Node {
public int data;
public Node next;
public Node[int d]
{
data = d;
next = null;
} // Constructor
}
/* method to create a simple linked list with 3 nodes*/
public static void Main[String[] args]
{
/* Start with the empty list. */
LinkedList llist = new LinkedList[];
llist.head = new Node[1];
Node second = new Node[2];
Node third = new Node[3];
/* Three nodes have been allocated dynamically.
We have references to these three blocks as head,
second and third
llist.head second third
| | |
| | |
+----+------+ +----+------+ +----+------+
| 1 | null | | 2 | null | | 3 | null |
+----+------+ +----+------+ +----+------+ */
llist.head.next = second; // Link first node with the second node
/* Now next of first Node refers to second. So they
both are linked.
llist.head second third
| | |
| | |
+----+------+ +----+------+ +----+------+
| 1 | o-------->| 2 | null | | 3 | null |
+----+------+ +----+------+ +----+------+ */
second.next = third; // Link second node with the third node
/* Now next of the second Node refers to third. So all three
nodes are linked.
llist.head second third
| | |
| | |
+----+------+ +----+------+ +----+------+
| 1 | o-------->| 2 | o-------->| 3 | null |
+----+------+ +----+------+ +----+------+ */
}
}
// This code has been contributed by 29AjayKumar
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Linked List Traversal
In the previous program, we have created a simple linked list with three nodes. Let us traverse the created list and print the data of each node. For traversal, let us write a general-purpose function printList[] that prints any given list.
What is a singly linked list?
A singly linked list is a type of linked list that is unidirectional, that is, it can be traversed in only one direction from head to the last node [tail].
Each element in a linked list is called a node. A single node contains data and a pointer to the next node which helps in maintaining the structure of the list.
The first node is called the head; it points to the first node of the list and helps us access every other element in the list. The last node, also sometimes called the tail, points to NULL which helps us in determining when the list ends.
You have to start somewhere, so we give the address of the first node a special name called HEAD. Also, the last node in the linked list can be identified because its next portion points to NULL.
Linked lists can be of multiple types: singly, doubly, and circular linked list. In this article, we will focus on the singly linked list. To learn about other types, visit Types of Linked List.
Note: You might have played the game Treasure Hunt, where each clue includes the information about the next clue. That is how the linked list operates.
Linked List
- Linked List can be defined as collection of objects called nodes that are randomly stored in the memory.
- A node contains two fields i.e. data stored at that particular address and the pointer which contains the address of the next node in the memory.
- The last node of the list contains pointer to the null.
Program to create and display a singly linked list
Explanation
In this program, we need to create a singly linked list and display all the nodes present in the list.
The singly linked list is a linear data structure in which each element of the list contains a pointer which points to the next element in the list. Each element in the singly linked list is called a node. Each node has two components: data and a pointer next which points to the next node in the list. The first node of the list is called as head, and the last node of the list is called a tail. The last node of the list contains a pointer to the null. Each node in the list can be accessed linearly by traversing through the list from head to tail.
Consider the above example; node 1 is the head of the list and node 4 is the tail of the list. Each node is connected in such a way that node 1 is pointing to node 2 which in turn pointing to node 3. Node 3 is again pointing to node 4. Node 4 is pointing to null as it is the last node of the list.
Algorithm
- Create a class Node which has two attributes: data and next. Next is a pointer to the next node.
- Create another class which has two attributes: head and tail.
- addNode[] will add a new node to the list:
- Create a new node.
- It first checks, whether the head is equal to null which means the list is empty.
- If the list is empty, both head and tail will point to the newly added node.
- If the list is not empty, the new node will be added to end of the list such that tail's next will point to the newly added node. This new node will become the new tail of the list.
- display[] will display the nodes present in the list:
- Define a node current which initially points to the head of the list.
- Traverse through the list till current points to null.
- Display each node by making current to point to node next to it in each iteration.
Solution
Python
Output:
Output:
JAVA
Output:
C#
Output:
PHP
Output: