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.
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.
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. 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 is NULL.
Each node in a list consists of at least two parts:
1) data
2) Pointer (Or Reference) to the next node
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.
C
// A linked list node struct Node { int data; struct Node* next; }; |
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CPP
class Node { public: int data; Node* next; }; |
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Java
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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Python
# 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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C#
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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First Simple Linked List in C Let us create a simple linked list with 3 nodes.
C++
// A simple CPP program to introduce // a linked list #include <bits/stdc++.h> 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 | | +---+---+ +---+---+ +----+------+ | 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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C
// A simple C program to introduce // a linked list #include <stdio.h> #include <stdlib.h> 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 | | +---+---+ +---+---+ +----+------+ | 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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Java
// 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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Python
# 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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C#
// 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.
We strongly recommend that you click here and practice it, before moving on to the solution.
C++
// A simple C++ program for traversal of a linked list #include <bits/stdc++.h> using namespace std; class Node { public: int data; Node* next; }; // This function prints contents of linked list // starting from the given node void printList(Node* n) { while (n != NULL) { cout << n->data << " "; n = n->next; } } // Driver code 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(); head->data = 1; // assign data in first node head->next = second; // Link first node with second second->data = 2; // assign data to second node second->next = third; third->data = 3; // assign data to third node third->next = NULL; printList(head); return 0; } // This is code is contributed by rathbhupendra |
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C
// A simple C program for traversal of a linked list #include <stdio.h> #include <stdlib.h> struct Node { int data; struct Node* next; }; // This function prints contents of linked list starting from // the given node void printList(struct Node* n) { while (n != NULL) { printf(" %d ", n->data); n = n->next; } } 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)); head->data = 1; // assign data in first node head->next = second; // Link first node with second second->data = 2; // assign data to second node second->next = third; third->data = 3; // assign data to third node third->next = NULL; printList(head); return 0; } |
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Java
// A simple Java program for traversal of 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 } /* This function prints contents of linked list starting from head */ public void printList() { Node n = head; while (n != null) { System.out.print(n.data + " "); n = n.next; } } /* 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); llist.head.next = second; // Link first node with the second node second.next = third; // Link first node with the second node llist.printList(); } } |
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Python3
# A simple Python program for traversal of 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 # This function prints contents of linked list # starting from head def printList(self): temp = self.head while (temp): print (temp.data) temp = temp.next # Code execution starts here if __name__=='__main__': # Start with the empty list llist = LinkedList() llist.head = Node(1) second = Node(2) third = Node(3) llist.head.next = second; # Link first node with second second.next = third; # Link second node with the third node llist.printList() |
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C#
// A simple C# program for traversal of 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 } /* This function prints contents of linked list starting from head */ public void printList() { Node n = head; while (n != null) { Console.Write(n.data + " "); n = n.next; } } /* 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); llist.head.next = second; // Link first node with the second node second.next = third; // Link first node with the second node llist.printList(); } } /* This code contributed by PrinciRaj1992 */ |
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Output:
1 2 3
Important Links :
- Practice MCQ Questions on Linked List
- Linked List Data Structure Page
- Coding Practice Questions on Linked List.
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