How to Find Length of a Linked List?

• A linked list is a linear data structure used for storing collections of data
• Successive elements are connected by pointers
• The last element points to NULL
• Each element is a separate object and is called a Node
• Each node in a linked list comprises of two parts
  • Data
  • Reference to Next Node

There are two ways to find the length of a linked list:

1. Iterative Approach
2. Recursive Approach

We will use the Linked list traversal to find the length of a linked list.

• Head Points to the First Node of The List.
• Initialize the count variable with value 0
• Initialize the temp variable with Head
• As we access each Node, the value of count variable is increased by 1.
• Stop The process when we reach null.
• Do not change the head reference.

Code in Java

package com.journaldev.ds;

public class MyLinkedList {

	public class Node {

		int data;

		Node next;

	}

	public Node head;
	public Node tail;
	public int size;

	public int getFirst() throws Exception {

		if (this.size == 0) {

			throw new Exception("linked list is empty");
		}

		return this.head.data;
	}

	public int getLast() throws Exception {

		if (this.size == 0) {

			throw new Exception("linked list is empty");
		}
		return this.tail.data;
	}

	public void display() {

		Node temp = this.head;
		while (temp != null) {
			System.out.println(temp.data + " ");
			temp = temp.next;
		}
	}

	public void addFirst(int item) {

		Node nn = new Node();

		nn.data = item;
		if (this.size == 0) {
			this.head = nn;
			this.tail = nn;
			this.size = this.size + 1;

		} else {

			nn.next = this.head;

			this.head = nn;

			this.size = this.size + 1;

		}

	}

	public int length() {

		Node temp = this.head;
		int count = 0;
		while (temp != null) {
			count++;
			temp = temp.next;
		}
		return count;
	}

	public static void main(String[] args) {

		MyLinkedList ll = new MyLinkedList();

		ll.addFirst(10);

		ll.addFirst(20);

		ll.addFirst(30);

		ll.addFirst(40);

		ll.addFirst(50);

		System.out.println("Length of Linked List is " + ll.length());

	}

}

Code in C

#include <stdio.h>

#include <stdlib.h>

/* A structure of linked list node */

struct node {

  int data;

  struct node *next;

} *head;

void initialize(){

    head = NULL;

}

/*

Inserts a node in front of a singly linked list.

*/

void insert(int num) {

    /* Create a new Linked List node */

    struct node* newNode = (struct node*) malloc(sizeof(struct node));

    newNode->data  = num;

    /* Next pointer of new node will point to head node of linked list  */

    newNode->next = head;

    /* make new node as the new head of linked list */

    head = newNode;

    printf("Inserted Element : %d\n", num);

}

int getLength(struct node *head){

    int length =0;

    while(head != NULL){

        head = head->next;

        length++;

    }

    return length;

}

/*

Prints a linked list from head node till the tail node

*/

void printLinkedList(struct node *nodePtr) {

  while (nodePtr != NULL) {

     printf("%d", nodePtr->data);

     nodePtr = nodePtr->next;

     if(nodePtr != NULL)

         printf("-->");

  }

}

int main() {

    initialize();

    /* Creating a linked List*/

    insert(8); 

    insert(3);

    insert(2);

    insert(7);

    insert(9);

    printf("\nLinked List\n");

    printLinkedList(head);

    printf("\nLinked List Length : %d", getLength(head));

    return 0;

}

Output

Base Case:

• Last Node points to Null value
• Return 0

Recursive Case:

• At each step update the Value of Current Node to the Next Node
• Call= 1+fun(curr.next)

Example: There are 3 elements in the linked list: LL1, LL2, and LL3. We will Observe What happens in the Memory Stack when the Recursive Call is made. MEMORY STACK:

The main function calls LL1, LL1 calls LL2, LL2 calls LL3, LL3 calls null value. As Null value is reached, we return from here. 0 is returned to LL3, LL3 returns 1 to LL2, LL2 returns 2 to LL1. LL1 finally returns 3 to the main function.

Code in Java

package com.journaldev.ds;

public class MyLinkedList {

    public class Node {

         int data;

         Node next;

    }

    public Node head;

    public Node tail;

    public int size;

    public int getfirst() throws Exception {

         if (this.size == 0) {

             throw new Exception("linked list is empty");

         }

         return this.head.data;

    }

    public int RemoveFirst() throws Exception {

         if (this.size == 0) {

             throw new Exception("LL is empty");

         }

         Node temp = this.head;

         if (this.size == 1) {

             this.head = null;

             this.tail = null;

             size = 0;

         } else {

             this.head = this.head.next;

             this.size--;

         }

         return temp.data;

    }

    public void addFirst(int item) {

         Node nn = new Node();

         nn.data = item;

         if (this.size == 0) {

             this.head = nn;

             this.tail = nn;

             this.size = this.size + 1;

         } else {

             nn.next = this.head;

             this.head = nn;

             this.size = this.size + 1;

         }

    }

    public int lengthUsingRecursiveApproach (){

         return lengthUsingRecursiveApproach(this.head);

    }

    private int lengthUsingRecursiveApproach(Node curr) {

         // TODO Auto-generated method stub

         if (curr == null) {

             return 0;

         }

         return 1 + lengthUsingRecursiveApproach (curr.next);

    }




    public static void main(String[] args) {

         MyLinkedList ll = new MyLinkedList();

         // insert elements into the Linked List

        ll.addFirst(10);

         ll.addFirst(20);

         ll.addFirst(30);

         ll.addFirst(40);

         ll.addFirst(50);

         // Length of List

         System.out.println("Recursive Approach length " + ll.lengthUsingRecursiveApproach(ll.head));

    }

}

Code in C

#include <stdio.h>

struct Node

{

    int data;

    struct Node* next;

};
void push(struct Node** head_ref, int new_data)
{

    struct Node* new_node =  (struct Node*) malloc(sizeof(struct Node));  

    new_node->data  = new_data;  

    /* link the old list of the new node */

    new_node->next = (*head_ref);

    (*head_ref)    = new_node;

}

int getCount(struct Node* head)

{

    // Base case

    if (head == NULL)

        return 0; 

    return 1 + getCount(head->next);

}

int main()

{

    struct Node* head = NULL;

    push(&head, 1);

    push(&head, 3);

    push(&head, 1);

    push(&head, 2);

    push(&head, 1);

    printf("count of nodes is %d", getCount(head));

    return 0;

}

Output

O(N) in both the recursive and iterative solution, as all we need, is a single traversal to know the length.