Find Length of a Linked List (Iterative and Recursive)

Write a function to count the number of nodes in a given singly linked list

Examples:

Input:

Input: 2->4->1->9->5->3->6
Output: 7 

An iterative approach for finding the length of the linked list:

Follow the given steps to solve the problem:

• Initialize count as 0 
• Initialize a node pointer, current = head.
• Do following while current is not NULL
  • current = current -> next
  • Increment count by 1.
• Return count 

Below is the implementation of the above approach:

C++

// Iterative C++ program to find length
// or count of nodes in a linked list
#include <bits/stdc++.h>
using namespace std;

/* Link list node */
class Node {
public:
    int data;
    Node* next;
};

/* Given a reference (pointer to pointer) to the head
of a list and an int, push a new node on the front
of the list. */
void push(Node** head_ref, int new_data)
{
    /* allocate node */
    Node* new_node = new Node();

    /* put in the data */
    new_node->data = new_data;

    /* link the old list of the new node */
    new_node->next = (*head_ref);

    /* move the head to point to the new node */
    (*head_ref) = new_node;
}

/* Counts no. of nodes in linked list */
int getCount(Node* head)
{
    int count = 0; // Initialize count
    Node* current = head; // Initialize current
    while (current != NULL) {
        count++;
        current = current->next;
    }
    return count;
}

/* Driver code*/
int main()
{
    /* Start with the empty list */
    Node* head = NULL;

    /* Use push() to construct below list
    1->2->1->3->1 */
    push(&head, 1);
    push(&head, 3);
    push(&head, 1);
    push(&head, 2);
    push(&head, 1);

    // Function call
    cout << "count of nodes is " << getCount(head);
    return 0;
}

// This is code is contributed by rathbhupendra

C

// Iterative C program to find length or count of nodes in a
// linked list
#include <stdio.h>
#include <stdlib.h>

/* Link list node */
struct Node {
    int data;
    struct Node* next;
};

/* Given a reference (pointer to pointer) to the head
  of a list and an int, push a new node on the front
  of the list. */
void push(struct Node** head_ref, int new_data)
{
    /* allocate node */
    struct Node* new_node
        = (struct Node*)malloc(sizeof(struct Node));

    /* put in the data  */
    new_node->data = new_data;

    /* link the old list of the new node */
    new_node->next = (*head_ref);

    /* move the head to point to the new node */
    (*head_ref) = new_node;
}

/* Counts no. of nodes in linked list */
int getCount(struct Node* head)
{
    int count = 0; // Initialize count
    struct Node* current = head; // Initialize current
    while (current != NULL) {
        count++;
        current = current->next;
    }
    return count;
}

/* Driver code*/
int main()
{
    /* Start with the empty list */
    struct Node* head = NULL;

    /* Use push() to construct below list
     1->2->1->3->1  */
    push(&head, 1);
    push(&head, 3);
    push(&head, 1);
    push(&head, 2);
    push(&head, 1);

    // Function call
    printf("count of nodes is %d", getCount(head));
    return 0;
}

Java

// Java program to count number of nodes in a linked list

/* Linked list Node*/
class Node {
    int data;
    Node next;
    Node(int d)
    {
        data = d;
        next = null;
    }
}

// Linked List class
class LinkedList {
    Node head; // head of list

    /* Inserts a new Node at front of the list. */
    public void push(int new_data)
    {
        /* 1 & 2: Allocate the Node &
                  Put in the data*/
        Node new_node = new Node(new_data);

        /* 3. Make next of new Node as head */
        new_node.next = head;

        /* 4. Move the head to point to new Node */
        head = new_node;
    }

    /* Returns count of nodes in linked list */
    public int getCount()
    {
        Node temp = head;
        int count = 0;
        while (temp != null) {
            count++;
            temp = temp.next;
        }
        return count;
    }

    // Driver code
    public static void main(String[] args)
    {
        /* Start with the empty list */
        LinkedList llist = new LinkedList();
        llist.push(1);
        llist.push(3);
        llist.push(1);
        llist.push(2);
        llist.push(1);

          // Function call
        System.out.println("Count of nodes is "
                           + llist.getCount());
    }
}

Python3

# A complete working Python program to find length of a
# Linked List iteratively

# 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 is in LinkedList class. It inserts
    # a new node at the beginning of Linked List.

    def push(self, new_data):

        # 1 & 2: Allocate the Node &
        #     Put in the data
        new_node = Node(new_data)

        # 3. Make next of new Node as head
        new_node.next = self.head

        # 4. Move the head to point to new Node
        self.head = new_node

    # This function counts number of nodes in Linked List
    # iteratively, given 'node' as starting node.

    def getCount(self):
        temp = self.head  # Initialise temp
        count = 0  # Initialise count

        # Loop while end of linked list is not reached
        while (temp):
            count += 1
            temp = temp.next
        return count


# Driver code
if __name__ == '__main__':
    llist = LinkedList()
    llist.push(1)
    llist.push(3)
    llist.push(1)
    llist.push(2)
    llist.push(1)

    # Function call
    print("Count of nodes is :", llist.getCount())

C#

// C# program to count number
// of nodes in a linked list
using System;

/* Linked list Node*/
public class Node {
    public int data;
    public Node next;
    public Node(int d)
    {
        data = d;
        next = null;
    }
}

// Linked List class
public class LinkedList {
    Node head; // head of list

    /* Inserts a new Node at front of the list. */
    public void push(int new_data)
    {
        /* 1 & 2: Allocate the Node &
                Put in the data*/
        Node new_node = new Node(new_data);

        /* 3. Make next of new Node as head */
        new_node.next = head;

        /* 4. Move the head to point to new Node */
        head = new_node;
    }

    /* Returns count of nodes in linked list */
    public int getCount()
    {
        Node temp = head;
        int count = 0;
        while (temp != null) {
            count++;
            temp = temp.next;
        }
        return count;
    }

    // Driver code
    public static void Main()
    {
        /* Start with the empty list */
        LinkedList llist = new LinkedList();
        llist.push(1);
        llist.push(3);
        llist.push(1);
        llist.push(2);
        llist.push(1);

          // Function call
        Console.WriteLine("Count of nodes is "
                          + llist.getCount());
    }
}

/* This code contributed by PrinciRaj1992 */

Javascript

<script>
// javascript program to count number of nodes in a linked list

/* Linked list Node*/
class Node {
    constructor(val) {
        this.data = val;
        this.next = null;
    }
}

// Linked List class
var head; // head of list

    /* Inserts a new Node at front of the list. */
     function push(new_data) {
        /*
         * 1 & 2: Allocate the Node & Put in the data
         */
        var new_node = new Node(new_data);

        /* 3. Make next of new Node as head */
        new_node.next = head;

        /* 4. Move the head to point to new Node */
        head = new_node;
    }

    /* Returns count of nodes in linked list */
     function getCount() {
        var temp = head;
        var count = 0;
        while (temp != null) {
            count++;
            temp = temp.next;
        }
        return count;
    }

    /*
     * Driver program to test above functions. Ideally this function should be in a
     * separate user class. It is kept here to keep code compact
     */

        /* Start with the empty list */
        push(1);
        push(3);
        push(1);
        push(2);
        push(1);

        document.write("Count of nodes is " + getCount());

// This code contributed by Rajput-Ji
</script>

Output

count of nodes is 5

Time complexity: O(N), Where N is the size of the linked list
Auxiliary Space: O(1), As constant extra space is used.

A recursive approach for finding the length of the linked list:

Follow the given steps to solve the problem:

• If the head is NULL, return 0.
• Otherwise, return 1 + getCount(head->next) 

Below is the implementation of the above approach:

C++

// Recursive C++ program to find length
// or count of nodes in a linked list
#include <bits/stdc++.h>
using namespace std;

/* Link list node */
class Node {
public:
    int data;
    Node* next;
};

/* Given a reference (pointer to pointer) to the head
of a list and an int, push a new node on the front
of the list. */
void push(Node** head_ref, int new_data)
{
    /* allocate node */
    Node* new_node = new Node();

    /* put in the data */
    new_node->data = new_data;

    /* link the old list of the new node */
    new_node->next = (*head_ref);

    /* move the head to point to the new node */
    (*head_ref) = new_node;
}

/* Recursively count number of nodes in linked list */
int getCount(Node* head)
{
    // Base Case
    if (head == NULL) {
        return 0;
    }
    // Count this node plus the rest of the list
    else {
        return 1 + getCount(head->next);
    }
}

/* Driver program to test count function*/
int main()
{
    /* Start with the empty list */
    Node* head = NULL;

    /* Use push() to construct below list
    1->2->1->3->1 */
    push(&head, 1);
    push(&head, 3);
    push(&head, 1);
    push(&head, 2);
    push(&head, 1);

    /* Check the count function */
    cout << "Count of nodes is " << getCount(head);
    return 0;
}

// This is code is contributed by rajsanghavi9

Java

// Recursive Java program to count number of nodes in
// a linked list

/* Linked list Node*/
class Node {
    int data;
    Node next;
    Node(int d)
    {
        data = d;
        next = null;
    }
}

// Linked List class
class LinkedList {
    Node head; // head of list

    /* Inserts a new Node at front of the list. */
    public void push(int new_data)
    {
        /* 1 & 2: Allocate the Node &
                  Put in the data*/
        Node new_node = new Node(new_data);

        /* 3. Make next of new Node as head */
        new_node.next = head;

        /* 4. Move the head to point to new Node */
        head = new_node;
    }

    /* Returns count of nodes in linked list */
    public int getCountRec(Node node)
    {
        // Base case
        if (node == null)
            return 0;

        // Count is this node plus rest of the list
        return 1 + getCountRec(node.next);
    }

    /* Wrapper over getCountRec() */
    public int getCount() { return getCountRec(head); }

    /* Driver program to test above functions. Ideally
       this function should be in a separate user class.
       It is kept here to keep code compact */
    public static void main(String[] args)
    {
        /* Start with the empty list */
        LinkedList llist = new LinkedList();
        llist.push(1);
        llist.push(3);
        llist.push(1);
        llist.push(2);
        llist.push(1);

        System.out.println("Count of nodes is "
                           + llist.getCount());
    }
}

Python3

# A complete working Python program to find length of a
# Linked List recursively

# 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 is in LinkedList class. It inserts
    # a new node at the beginning of Linked List.

    def push(self, new_data):

        # 1 & 2: Allocate the Node &
        #        Put in the data
        new_node = Node(new_data)

        # 3. Make next of new Node as head
        new_node.next = self.head

        # 4. Move the head to point to new Node
        self.head = new_node

    # This function counts number of nodes in Linked List
    # recursively, given 'node' as starting node.
    def getCountRec(self, node):
        if (not node):  # Base case
            return 0
        else:
            return 1 + self.getCountRec(node.next)

    # A wrapper over getCountRec()
    def getCount(self):
        return self.getCountRec(self.head)


# Code execution starts here
if __name__ == '__main__':
    llist = LinkedList()
    llist.push(1)
    llist.push(3)
    llist.push(1)
    llist.push(2)
    llist.push(1)
    print('Count of nodes is :', llist.getCount())

C#

// Recursive C# program to count number of nodes in
// a linked list
using System;

/* Linked list Node*/
public class Node {
    public int data;
    public Node next;
    public Node(int d)
    {
        data = d;
        next = null;
    }
}

// Linked List class
public class LinkedList {
    Node head; // head of list

    /* Inserts a new Node at front of the list. */
    public void push(int new_data)
    {
        /* 1 & 2: Allocate the Node &
                Put in the data*/
        Node new_node = new Node(new_data);

        /* 3. Make next of new Node as head */
        new_node.next = head;

        /* 4. Move the head to point to new Node */
        head = new_node;
    }

    /* Returns count of nodes in linked list */
    public int getCountRec(Node node)
    {
        // Base case
        if (node == null)
            return 0;

        // Count is this node plus rest of the list
        return 1 + getCountRec(node.next);
    }

    /* Wrapper over getCountRec() */
    public int getCount() { return getCountRec(head); }

    /* Driver program to test above functions. Ideally
    this function should be in a separate user class.
    It is kept here to keep code compact */
    public static void Main(String[] args)
    {
        /* Start with the empty list */
        LinkedList llist = new LinkedList();
        llist.push(1);
        llist.push(3);
        llist.push(1);
        llist.push(2);
        llist.push(1);

        Console.WriteLine("Count of nodes is "
                          + llist.getCount());
    }
}

// This code is contributed 29AjayKumar

Javascript

<script>
// Recursive javascript program to count number of nodes in
// a linked list

/* Linked list Node*/
class Node {
    constructor(val) {
            this.data = val;
            this.next = null;
        }
    }

    // Linked List class
    var head; // head of list

    /* Inserts a new Node at front of the list. */
    function push( new_data) {
        /*
         * 1 & 2: Allocate the Node & Put in the data
         */
        var new_node = new Node(new_data);

        /* 3. Make next of new Node as head */
        new_node.next = head;

        /* 4. Move the head to point to new Node */
        head = new_node;
    }

    /* Returns count of nodes in linked list */
     function getCountRec(node) {
        // Base case
        if (node == null)
            return 0;

        // Count is this node plus rest of the list
        return 1 + getCountRec(node.next);
    }

    /* Wrapper over getCountRec() */
     function getCount() {
        return getCountRec(head);
    }

    /*
     * Driver program to test above functions. Ideally this function should be in a
     * separate user class. It is kept here to keep code compact
     */

        /* Start with the empty list */

        push(1);
        push(3);
        push(1);
        push(2);
        push(1);

        document.write("Count of nodes is " + getCount());

// This code contributed by gauravrajput1
</script>

Output

Count of nodes is 5

Time Complexity: O(N), As we are traversing the linked list only once.
Auxiliary Space: O(N), Extra space is used in the recursion call stack.

Recursive approach for finding the length of the linked list using constant space:

To solve the problem follow the below idea:

The above recursive approach can be modified to make it a tail recursive function and thus our auxiliary space will become O(1)

Below is the implementation of the above approach:

C++

// Tail Recursive C++ program to find length
// or count of nodes in a linked list
#include <bits/stdc++.h>
using namespace std;

/* Link list node */
class Node {
public:
    int data;
    Node* next;
};

/* Given a reference (pointer to pointer) to the head
of a list and an int, push a new node on the front
of the list. */
void push(Node** head_ref, int new_data)
{
    /* allocate node */
    Node* new_node = new Node();

    /* put in the data */
    new_node->data = new_data;

    /* link the old list of the new node */
    new_node->next = (*head_ref);

    /* move the head to point to the new node */
    (*head_ref) = new_node;
}
// A tail recursive function to count the nodes of a linked
// list
// default value of the count is used as zero
int getCount(Node* head, int count = 0)
{
    // base case
    if (head == NULL)
        return count;
    // move the pointer to next node and increase the count
    return getCount(head->next, count + 1);
}

/* Driver code*/
int main()
{
    /* Start with the empty list */
    Node* head = NULL;

    /* Use push() to construct below list
    1->2->1->3->1 */
    push(&head, 1);
    push(&head, 3);
    push(&head, 1);
    push(&head, 2);
    push(&head, 1);

    // Function call
    cout << "Count of nodes is " << getCount(head);
    return 0;
}

// This is code is contributed by Abhijeet Kumar

Java

// Tail Recursive Java program to count number of nodes in
// a linked list

/* Linked list Node*/
class Node {
    int data;
    Node next;
    Node(int d)
    {
        data = d;
        next = null;
    }
}

// Linked List class
class LinkedList {
    Node head; // head of list

    /* Inserts a new Node at front of the list. */
    public void push(int new_data)
    {
        /* 1 & 2: Allocate the Node &
                  Put in the data*/
        Node new_node = new Node(new_data);

        /* 3. Make next of new Node as head */
        new_node.next = head;

        /* 4. Move the head to point to new Node */
        head = new_node;
    }

    /* Returns count of nodes in linked list */
    public int getCountRec(Node node, int count)
    {
        // Base case
        if (node == null)
            return count;

        // Move to the next node and increase count
        return getCountRec(node.next, 1 + count);
    }

    /* Wrapper over getCountRec() */
    public int getCount() { return getCountRec(head, 0); }

    // Driver code
    public static void main(String[] args)
    {
        LinkedList llist = new LinkedList();
        llist.push(1);
        llist.push(3);
        llist.push(1);
        llist.push(2);
        llist.push(1);

          // Function call
        System.out.println("Count of nodes is "
                           + llist.getCount());
    }
}

// This code was contributed by Abhijeet
// Kumar(abhijeet19403)

Python3

# A complete working Python3 program to find length of a
# Linked List using Tail recursion.

# 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 is in LinkedList class. It inserts
    # a new node at the beginning of Linked List.

    def push(self, new_data):

        # 1 & 2: Allocate the Node &
        #     Put in the data
        new_node = Node(new_data)

        # 3. Make next of new Node as head
        new_node.next = self.head

        # 4. Move the head to point to new Node
        self.head = new_node

    # This function counts number of nodes in Linked List
    # recursively, given 'node' as starting node using Tail Recursion.
    def getCountRec(self, node, count=0):
        if (not node):  # Base case
            return count
        else:
            return self.getCountRec(node.next, count+1)

    # A wrapper over getCountRec()
    def getCount(self):
        return self.getCountRec(self.head)


# Driver code
if __name__ == '__main__':
    llist = LinkedList()
    llist.push(1)
    llist.push(3)
    llist.push(1)
    llist.push(2)
    llist.push(1)

    # Function call
    print('Count of nodes is :', llist.getCount())

    # This code is contributed by garinesrija.

C#

// Tail Recursive C# program to count number of nodes in
// a linked list
using System;

/* Linked list Node*/
public class Node {
    public int data;
    public Node next;
    public Node(int d)
    {
        data = d;
        next = null;
    }
}

// Linked List class
public class LinkedList {
    Node head; // head of list

    /* Inserts a new Node at front of the list. */
    public void push(int new_data)
    {
        /* 1 & 2: Allocate the Node &
                Put in the data*/
        Node new_node = new Node(new_data);

        /* 3. Make next of new Node as head */
        new_node.next = head;

        /* 4. Move the head to point to new Node */
        head = new_node;
    }

    /* Returns count of nodes in linked list */
    public int getCountRec(Node node, int count)
    {
        // Base case
        if (node == null)
            return count;

        // Move the pointer to next node and increase the
        // count
        return getCountRec(node.next, 1 + count);
    }

    /* Wrapper over getCountRec() */
    public int getCount() { return getCountRec(head, 0); }

    // Driver code
    public static void Main(String[] args)
    {
        LinkedList llist = new LinkedList();
        llist.push(1);
        llist.push(3);
        llist.push(1);
        llist.push(2);
        llist.push(1);

          // Function call
        Console.WriteLine("Count of nodes is "
                          + llist.getCount());
    }
}

// This code is contributed Abhijeet Kumar(abhijeet19403)

Javascript

<script>
// Tail Recursive javascript program to count number of nodes in
// a linked list

/* Linked list Node*/
class Node {
    constructor(val) {
            this.data = val;
            this.next = null;
        }
    }

    // Linked List class
    var head; // head of list

    /* Inserts a new Node at front of the list. */
    function push( new_data) {
        /*
         * 1 & 2: Allocate the Node & Put in the data
         */
        var new_node = new Node(new_data);

        /* 3. Make next of new Node as head */
        new_node.next = head;

        /* 4. Move the head to point to new Node */
        head = new_node;
    }

    /* Returns count of nodes in linked list */
     function getCountRec(node,var count) {
        // Base case
        if (node == null)
            return count;

        // Move the pointer to next node and increase the count
        return getCountRec(node.next,count+1);
    }

    /* Wrapper over getCountRec() */
     function getCount() {
        return getCountRec(head,0);
    }

    /*
     * Driver program to test above functions. Ideally this function should be in a
     * separate user class. It is kept here to keep code compact
     */

        /* Start with the empty list */

        push(1);
        push(3);
        push(1);
        push(2);
        push(1);

        document.write("Count of nodes is " + getCount());

// This code contributed by Abhijeet Kumar(abhijeet19403)
</script>

Output

Count of nodes is 5

Time Complexity: O(N), As we are traversing the list only once.
Auxiliary Space: O(1), As we are using the tail recursive function, no extra space is used in the function call stack.