Count duplicates in a given linked list

Given a linked list. The task is to count the number of duplicate nodes in the linked list.
Examples:

Input: 5 -> 7 -> 5 -> 1 -> 7 -> NULL
Output: 2

Input: 5 -> 7 -> 8 -> 7 -> 1 -> NULL
Output: 1

Simple Approach: We traverse the whole linked list. For each node we check in the remaining list whether the duplicate node exists or not. If it does then we increment the count.

Below is the implementation of the above approach:

C++



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// C++ implementation of the approach
#include <iostream>
#include <unordered_set>
using namespace std;
  
// Representation of node
struct Node {
    int data;
    Node* next;
};
  
  
// Function to insert a node at the beginning
void insert(Node** head, int item)
{
    Node* temp = new Node();
    temp->data = item;
    temp->next = *head;
    *head = temp;
}
  
// Function to count the number of
// duplicate nodes in the linked list
int countNode(Node* head)
{
    int count = 0;
  
    while (head->next != NULL) {
  
        // Starting from the next node
        Node *ptr = head->next;
        while (ptr != NULL) {
  
            // If some duplicate node is found
            if (head->data == ptr->data) {
                count++;
                break;
            }
            ptr = ptr->next;
        }
  
        head = head->next;
    }
  
    // Return the count of duplicate nodes
    return count;
}
  
// Driver code
int main()
{
    Node* head = NULL;
    insert(&head, 5);
    insert(&head, 7);
    insert(&head, 5);
    insert(&head, 1);
    insert(&head, 7);
  
    cout << countNode(head);
  
    return 0;
}

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Java

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// Java implementation of the approach 
class GFG
{
      
// Representation of node 
static class Node 
    int data; 
    Node next; 
}; 
  
  
// Function to insert a node at the beginning 
static Node insert(Node head, int item) 
    Node temp = new Node(); 
    temp.data = item; 
    temp.next = head; 
    head = temp;
    return head;
  
// Function to count the number of 
// duplicate nodes in the linked list 
static int countNode(Node head) 
    int count = 0
  
    while (head.next != null
    
  
        // Starting from the next node 
        Node ptr = head.next; 
        while (ptr != null
        
  
            // If some duplicate node is found 
            if (head.data == ptr.data) 
            
                count++; 
                break
            
            ptr = ptr.next; 
        
  
        head = head.next; 
    
  
    // Return the count of duplicate nodes 
    return count; 
  
// Driver code 
public static void main(String args[])
    Node head = null
    head = insert(head, 5); 
    head = insert(head, 7); 
    head = insert(head, 5); 
    head = insert(head, 1); 
    head = insert(head, 7); 
  
    System.out.println( countNode(head)); 
}
  
// This code is contributed by Arnab Kundu

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Python3

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# Python3 implementation of the approach
import math
  
# Representation of node
class Node: 
    def __init__(self, data): 
        self.data = data 
        self.next = None
  
# Function to push a node at the beginning
def push(head, item): 
    temp = Node(item); 
    temp.data = item; 
    temp.next = head; 
    head = temp;
    return head;
          
# Function to count the number of
# duplicate nodes in the linked list
def countNode(head):
    count = 0
    while (head.next != None):
          
        # print(1)
        # Starting from the next node
        ptr = head.next
        while (ptr != None):
              
            # print(2)
            # If some duplicate node is found
            if (head.data == ptr.data):
                count = count + 1
                break
            ptr = ptr.next
        head = head.next
          
    # Return the count of duplicate nodes
    return count
  
# Driver code
if __name__=='__main__'
  
    head = None;
    head = push(head, 5)
    head = push(head, 7)
    head = push(head, 5)
    head = push(head, 1)
    head = push(head, 7)
    print(countNode(head))
  
# This code is contributed by Srathore

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C#

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// C# implementation of the approach 
using System;
      
class GFG
{
      
// Representation of node 
public class Node 
    public int data; 
    public Node next; 
}; 
  
  
// Function to insert a node at the beginning 
static Node insert(Node head, int item) 
    Node temp = new Node(); 
    temp.data = item; 
    temp.next = head; 
    head = temp;
    return head;
  
// Function to count the number of 
// duplicate nodes in the linked list 
static int countNode(Node head) 
    int count = 0; 
  
    while (head.next != null
    
  
        // Starting from the next node 
        Node ptr = head.next; 
        while (ptr != null
        
  
            // If some duplicate node is found 
            if (head.data == ptr.data) 
            
                count++; 
                break
            
            ptr = ptr.next; 
        
  
        head = head.next; 
    
  
    // Return the count of duplicate nodes 
    return count; 
  
// Driver code 
public static void Main(String []args)
    Node head = null
    head = insert(head, 5); 
    head = insert(head, 7); 
    head = insert(head, 5); 
    head = insert(head, 1); 
    head = insert(head, 7); 
  
    Console.WriteLine( countNode(head)); 
}
}
  
// This code is contributed by Rajput-Ji

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Output:

2

Time Complexity : O(n*n)

Efficient Approach: The idea is to use hashing

C++

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// C++ implementation of the approach
#include <iostream>
#include <unordered_set>
using namespace std;
  
// Representation of node
struct Node {
    int data;
    Node* next;
};
  
// Function to insert a node at the beginning
void insert(Node** head, int item)
{
    Node* temp = new Node();
    temp->data = item;
    temp->next = *head;
    *head = temp;
}
  
// Function to count the number of
// duplicate nodes in the linked list
int countNode(Node* head)
{
    if (head == NULL)
       return 0;;
  
    // Create a hash table insert head
    unordered_set<int> s;
    s.insert(head->data);
  
    // Traverse through remaining nodes    
    int count = 0;
    for (Node *curr=head->next; curr != NULL; curr=curr->next) {
        if (s.find(curr->data) != s.end())
             count++; 
  
        s.insert(curr->data);
    }
  
    // Return the count of duplicate nodes
    return count;
}
  
// Driver code
int main()
{
    Node* head = NULL;
    insert(&head, 5);
    insert(&head, 7);
    insert(&head, 5);
    insert(&head, 1);
    insert(&head, 7);
  
    cout << countNode(head);
  
    return 0;
}

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Java

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// Java implementation of the approach
import java.util.HashSet;
  
class GFG 
{
  
// Representation of node
static class Node
{
    int data;
    Node next;
};
static Node head;
  
// Function to insert a node at the beginning
static void insert(Node ref_head, int item)
{
    Node temp = new Node();
    temp.data = item;
    temp.next = ref_head;
    head = temp;
      
}
  
// Function to count the number of
// duplicate nodes in the linked list
static int countNode(Node head)
{
    if (head == null)
    return 0;;
  
    // Create a hash table insert head
    HashSet<Integer>s = new HashSet<>();
    s.add(head.data);
  
    // Traverse through remaining nodes 
    int count = 0;
    for (Node curr=head.next; curr != null; curr=curr.next) 
    {
        if (s.contains(curr.data))
            count++; 
  
        s.add(curr.data);
    }
  
    // Return the count of duplicate nodes
    return count;
}
  
// Driver code
public static void main(String[] args) 
{
  
    insert(head, 5);
    insert(head, 7);
    insert(head, 5);
    insert(head, 1);
    insert(head, 7);
  
    System.out.println(countNode(head));
}
}
  
// This code is contributed by Princi Singh

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Python

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# Python3 implementation of the approach 
  
# Node of a linked list 
class Node: 
    def __init__(self, data = None, next = None): 
        self.next = next
        self.data = data 
  
head = None
  
# Function to insert a node at the beginning
def insert(ref_head, item):
    global head
    temp = Node()
    temp.data = item
    temp.next = ref_head
    head = temp
      
# Function to count the number of
# duplicate nodes in the linked list
def countNode(head):
  
    if (head == None):
        return 0
  
    # Create a hash table insert head
    s = set()
    s.add(head.data)
  
    # Traverse through remaining nodes 
    count = 0
    curr = head.next
    while ( curr != None ) :
        if (curr.data in s):
            count = count + 1
  
        s.add(curr.data)
        curr = curr.next
  
    # Return the count of duplicate nodes
    return count
  
# Driver code
insert(head, 5)
insert(head, 7)
insert(head, 5)
insert(head, 1)
insert(head, 7)
  
print(countNode(head))
  
# This code is contributed by Arnab Kundu

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C#

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// C# implementation of the approach
using System;
using System.Collections.Generic;
      
class GFG 
{
  
// Representation of node
public class Node
{
    public int data;
    public Node next;
};
static Node head;
  
// Function to insert a node at the beginning
static void insert(Node ref_head, int item)
{
    Node temp = new Node();
    temp.data = item;
    temp.next = ref_head;
    head = temp;
      
}
  
// Function to count the number of
// duplicate nodes in the linked list
static int countNode(Node head)
{
    if (head == null)
    return 0;;
  
    // Create a hash table insert head
    HashSet<int>s = new HashSet<int>();
    s.Add(head.data);
  
    // Traverse through remaining nodes 
    int count = 0;
    for (Node curr=head.next; curr != null; curr=curr.next) 
    {
        if (s.Contains(curr.data))
            count++; 
  
        s.Add(curr.data);
    }
  
    // Return the count of duplicate nodes
    return count;
}
  
// Driver code
public static void Main(String[] args) 
{
  
    insert(head, 5);
    insert(head, 7);
    insert(head, 5);
    insert(head, 1);
    insert(head, 7);
  
    Console.WriteLine(countNode(head));
}
}
  
// This code is contributed by 29AjayKumar

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Output:

2

Time Complexity : O(n)

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