Data Structures

Length of Linked List

Following is an algorithm to count number of nodes in a given singly linked list.

For example, the function should return 5 for linked list 1->3->1->2->1.

Iterative Solution

1) Initialize count as 0 

2) Initialize a node pointer, current = head.

3) Do following while current is not NULL
a) current = current -> next
b) count++;

4) Return count

Following are C/C++, Java and Python implementations of above algorithm to find count of nodes.

// 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 off 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 program to test count function*/
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);

    /* Check the count function */
    printf("count of nodes is %d", getCount(head));
    return 0;
}

// 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 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());
    }
}

# 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


# 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()

Output:

count of nodes is 5

Recursive Solution

int getCount(head) 

1) If head is NULL, return 0.

2) Else return 1 + getCount(head->next)

Following are C/C++, Java and Python implementations of above algorithm to find count of nodes.

// Recursive 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 off the new node */
    new_node->next = (*head_ref);

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

/* Counts the no. of occurences of a node
   (search_for) in a linked list (head)*/
int getCount(struct node* head)
{
    // Base case
    if (head == NULL)
        return 0;

    // count is 1 + count of remaining list
    return 1 + getCount(head->next);
}

/* Driver program to test count function*/
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);

    /* Check the count function */
    printf("count of nodes is %d", getCount(head));
    return 0;
}

// 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());
    }
}

# 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()

Output:

count of nodes is 5


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