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Remove duplicates from an unsorted linked list

  • Difficulty Level : Easy
  • Last Updated : 01 Oct, 2021

Write a removeDuplicates() function that takes a list and deletes any duplicate nodes from the list. The list is not sorted. 
For example if the linked list is 12->11->12->21->41->43->21 then removeDuplicates() should convert the list to 12->11->21->41->43. 
 

METHOD 1 (Using two loops) 
This is the simple way where two loops are used. Outer loop is used to pick the elements one by one and the inner loop compares the picked element with the rest of the elements. 
Thanks to Gaurav Saxena for his help in writing this code.

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C++




/* C++ Program to remove duplicates in an unsorted
   linked list */
#include <bits/stdc++.h>
using namespace std;
 
/* A linked list node */
struct Node {
    int data;
    struct Node* next;
};
 
// Utility function to create a new Node
struct Node* newNode(int data)
{
    Node* temp = new Node;
    temp->data = data;
    temp->next = NULL;
    return temp;
}
 
/* Function to remove duplicates from a
   unsorted linked list */
void removeDuplicates(struct Node* start)
{
    struct Node *ptr1, *ptr2, *dup;
    ptr1 = start;
 
    /* Pick elements one by one */
    while (ptr1 != NULL && ptr1->next != NULL) {
        ptr2 = ptr1;
 
        /* Compare the picked element with rest
           of the elements */
        while (ptr2->next != NULL) {
            /* If duplicate then delete it */
            if (ptr1->data == ptr2->next->data) {
                /* sequence of steps is important here */
                ptr2->next = ptr2->next->next;
                delete (dup);
            }
            else /* This is tricky */
                ptr2 = ptr2->next;
        }
        ptr1 = ptr1->next;
    }
}
 
/* Function to print nodes in a given linked list */
void printList(struct Node* node)
{
    while (node != NULL) {
        printf("%d ", node->data);
        node = node->next;
    }
}
 
/* Driver program to test above function */
int main()
{
    /* The constructed linked list is:
     10->12->11->11->12->11->10*/
    struct Node* start = newNode(10);
    start->next = newNode(12);
    start->next->next = newNode(11);
    start->next->next->next = newNode(11);
    start->next->next->next->next = newNode(12);
    start->next->next->next->next->next = newNode(11);
    start->next->next->next->next->next->next = newNode(10);
 
    printf("Linked list before removing duplicates ");
    printList(start);
 
    removeDuplicates(start);
 
    printf("\nLinked list after removing duplicates ");
    printList(start);
 
    return 0;
}

Java




// Java program to remove duplicates from unsorted
// linked list
 
class LinkedList {
 
    static Node head;
 
    static class Node {
 
        int data;
        Node next;
 
        Node(int d)
        {
            data = d;
            next = null;
        }
    }
 
    /* Function to remove duplicates from an
       unsorted linked list */
    void remove_duplicates()
    {
        Node ptr1 = null, ptr2 = null, dup = null;
        ptr1 = head;
 
        /* Pick elements one by one */
        while (ptr1 != null && ptr1.next != null) {
            ptr2 = ptr1;
 
            /* Compare the picked element with rest
                of the elements */
            while (ptr2.next != null) {
 
                /* If duplicate then delete it */
                if (ptr1.data == ptr2.next.data) {
 
                    /* sequence of steps is important here
                     */
                    ptr2.next = ptr2.next.next;
                    System.gc();
                }
                else /* This is tricky */ {
                    ptr2 = ptr2.next;
                }
            }
            ptr1 = ptr1.next;
        }
    }
 
    void printList(Node node)
    {
        while (node != null) {
            System.out.print(node.data + " ");
            node = node.next;
        }
    }
 
    public static void main(String[] args)
    {
        LinkedList list = new LinkedList();
        list.head = new Node(10);
        list.head.next = new Node(12);
        list.head.next.next = new Node(11);
        list.head.next.next.next = new Node(11);
        list.head.next.next.next.next = new Node(12);
        list.head.next.next.next.next.next = new Node(11);
        list.head.next.next.next.next.next.next
            = new Node(10);
 
        System.out.println(
            "Linked List before removing duplicates : \n ");
        list.printList(head);
 
        list.remove_duplicates();
        System.out.println("");
        System.out.println(
            "Linked List after removing duplicates : \n ");
        list.printList(head);
    }
}
// This code has been contributed by Mayank Jaiswal

Python3




# Python3 program to remove duplicates
# from unsorted linked list
 
 
class Node():
 
    def __init__(self, data):
 
        self.data = data
        self.next = None
 
 
class LinkedList():
 
    def __init__(self):
 
        # Head of list
        self.head = None
 
    def remove_duplicates(self):
 
        ptr1 = None
        ptr2 = None
        dup = None
        ptr1 = self.head
 
        # Pick elements one by one
        while (ptr1 != None and ptr1.next != None):
 
            ptr2 = ptr1
 
            # Compare the picked element with rest
            # of the elements
            while (ptr2.next != None):
 
                # If duplicate then delete it
                if (ptr1.data == ptr2.next.data):
 
                    # Sequence of steps is important here
                    dup = ptr2.next
                    ptr2.next = ptr2.next.next
                else:
                    ptr2 = ptr2.next
 
            ptr1 = ptr1.next
 
    # Function to print nodes in a
    # given linked list
    def printList(self):
        temp = self.head
 
        while(temp != None):
            print(temp.data, end=" ")
            temp = temp.next
 
        print()
 
 
# Driver code
list = LinkedList()
list.head = Node(10)
list.head.next = Node(12)
list.head.next.next = Node(11)
list.head.next.next.next = Node(11)
list.head.next.next.next.next = Node(12)
list.head.next.next.next.next.next = Node(11)
list.head.next.next.next.next.next.next = Node(10)
 
print("Linked List before removing duplicates :")
list.printList()
list.remove_duplicates()
print()
print("Linked List after removing duplicates :")
list.printList()
 
# This code is contributed by maheshwaripiyush9

C#




// C# program to remove duplicates from unsorted
// linked list
using System;
class List_ {
 
    Node head;
 
    class Node {
 
        public int data;
        public Node next;
 
        public
 
            Node(int d)
        {
            data = d;
            next = null;
        }
    }
 
    /* Function to remove duplicates from an
         unsorted linked list */
    void remove_duplicates()
    {
        Node ptr1 = null, ptr2 = null, dup = null;
        ptr1 = head;
 
        /* Pick elements one by one */
        while (ptr1 != null && ptr1.next != null) {
            ptr2 = ptr1;
 
            /* Compare the picked element with rest
                      of the elements */
            while (ptr2.next != null) {
 
                /* If duplicate then delete it */
                if (ptr1.data == ptr2.next.data) {
 
                    /* sequence of steps is important here
                     */
                    dup = ptr2.next;
                    ptr2.next = ptr2.next.next;
                }
                else /* This is tricky */
                {
                    ptr2 = ptr2.next;
                }
            }
            ptr1 = ptr1.next;
        }
    }
 
    void printList(Node node)
    {
        while (node != null) {
            Console.Write(node.data + " ");
            node = node.next;
        }
    }
 
    // Driver Code
    public static void Main(String[] args)
    {
        List_ list = new List_();
        list.head = new Node(10);
        list.head.next = new Node(12);
        list.head.next.next = new Node(11);
        list.head.next.next.next = new Node(11);
        list.head.next.next.next.next = new Node(12);
        list.head.next.next.next.next.next = new Node(11);
        list.head.next.next.next.next.next.next
            = new Node(10);
 
        Console.WriteLine(
            "Linked List_ before removing duplicates : \n ");
        list.printList(list.head);
 
        list.remove_duplicates();
        Console.WriteLine("");
        Console.WriteLine(
            "Linked List_ after removing duplicates : \n ");
        list.printList(list.head);
    }
}
 
// This code is contributed by gauravrajput1

Javascript




<script>
// javascript program to remove duplicates from unsorted
// linked list
 
 
 
    var head;
 
    class Node {
        constructor(val) {
            this.data = val;
            this.next = null;
        }
    }
    /*
     * Function to remove duplicates from an unsorted linked list
     */
    function remove_duplicates() {
        var ptr1 = null, ptr2 = null, dup = null;
        ptr1 = head;
 
        /* Pick elements one by one */
        while (ptr1 != null && ptr1.next != null) {
            ptr2 = ptr1;
 
            /*
             * Compare the picked element with rest of the elements
             */
            while (ptr2.next != null) {
 
                /* If duplicate then delete it */
                if (ptr1.data == ptr2.next.data) {
 
                    /* sequence of steps is important here */
                    dup = ptr2.next;
                    ptr2.next = ptr2.next.next;
     
                } else /* This is tricky */ {
                    ptr2 = ptr2.next;
                }
            }
            ptr1 = ptr1.next;
        }
    }
 
    function printList( node) {
        while (node != null) {
            document.write(node.data + " ");
            node = node.next;
        }
    }
 
     
        head = new Node(10);
        head.next = new Node(12);
        head.next.next = new Node(11);
        head.next.next.next = new Node(11);
        head.next.next.next.next = new Node(12);
        head.next.next.next.next.next = new Node(11);
        head.next.next.next.next.next.next = new Node(10);
 
        document.write("Linked List before removing duplicates : <br/> ");
        printList(head);
 
        remove_duplicates();
        document.write("<br/>");
        document.write("Linked List after removing duplicates : <br/> ");
        printList(head);
 
// This code contributed by aashish1995
</script>
Output



Linked list before removing duplicates 10 12 11 11 12 11 10 
Linked list after removing duplicates 10 12 11 

Time Complexity: O(n^2)

METHOD 2 (Use Sorting) 
In general, Merge Sort is the best-suited sorting algorithm for sorting linked lists efficiently. 
1) Sort the elements using Merge Sort. We will soon be writing a post about sorting a linked list. O(nLogn) 
2) Remove duplicates in linear time using the algorithm for removing duplicates in sorted Linked List. O(n) 
Please note that this method doesn’t preserve the original order of elements.
Time Complexity: O(nLogn)

METHOD 3 (Use Hashing) 
We traverse the link list from head to end. For every newly encountered element, we check whether it is in the hash table: if yes, we remove it; otherwise we put it in the hash table.

C++




/* C++ Program to remove duplicates in an unsorted
   linked list */
#include <bits/stdc++.h>
using namespace std;
 
/* A linked list node */
struct Node {
    int data;
    struct Node* next;
};
 
// Utility function to create a new Node
struct Node* newNode(int data)
{
    Node* temp = new Node;
    temp->data = data;
    temp->next = NULL;
    return temp;
}
 
/* Function to remove duplicates from a
   unsorted linked list */
void removeDuplicates(struct Node* start)
{
    // Hash to store seen values
    unordered_set<int> seen;
 
    /* Pick elements one by one */
    struct Node* curr = start;
    struct Node* prev = NULL;
    while (curr != NULL) {
        // If current value is seen before
        if (seen.find(curr->data) != seen.end()) {
            prev->next = curr->next;
            delete (curr);
        }
        else {
            seen.insert(curr->data);
            prev = curr;
        }
        curr = prev->next;
    }
}
 
/* Function to print nodes in a given linked list */
void printList(struct Node* node)
{
    while (node != NULL) {
        printf("%d ", node->data);
        node = node->next;
    }
}
 
/* Driver program to test above function */
int main()
{
    /* The constructed linked list is:
     10->12->11->11->12->11->10*/
    struct Node* start = newNode(10);
    start->next = newNode(12);
    start->next->next = newNode(11);
    start->next->next->next = newNode(11);
    start->next->next->next->next = newNode(12);
    start->next->next->next->next->next = newNode(11);
    start->next->next->next->next->next->next = newNode(10);
 
    printf("Linked list before removing duplicates : \n");
    printList(start);
 
    removeDuplicates(start);
 
    printf("\nLinked list after removing duplicates : \n");
    printList(start);
 
    return 0;
}

Java




// Java program to remove duplicates
// from unsorted linkedlist
 
import java.util.HashSet;
 
public class removeDuplicates {
    static class node {
        int val;
        node next;
 
        public node(int val) { this.val = val; }
    }
 
    /* Function to remove duplicates from a
       unsorted linked list */
    static void removeDuplicate(node head)
    {
        // Hash to store seen values
        HashSet<Integer> hs = new HashSet<>();
 
        /* Pick elements one by one */
        node current = head;
        node prev = null;
        while (current != null) {
            int curval = current.val;
 
            // If current value is seen before
            if (hs.contains(curval)) {
                prev.next = current.next;
            }
            else {
                hs.add(curval);
                prev = current;
            }
            current = current.next;
        }
    }
 
    /* Function to print nodes in a given linked list */
    static void printList(node head)
    {
        while (head != null) {
            System.out.print(head.val + " ");
            head = head.next;
        }
    }
 
    public static void main(String[] args)
    {
        /* The constructed linked list is:
         10->12->11->11->12->11->10*/
        node start = new node(10);
        start.next = new node(12);
        start.next.next = new node(11);
        start.next.next.next = new node(11);
        start.next.next.next.next = new node(12);
        start.next.next.next.next.next = new node(11);
        start.next.next.next.next.next.next = new node(10);
 
        System.out.println(
            "Linked list before removing duplicates :");
        printList(start);
 
        removeDuplicate(start);
 
        System.out.println(
            "\nLinked list after removing duplicates :");
        printList(start);
    }
}
 
// This code is contributed by Rishabh Mahrsee

Python3




# Python3 program to remove duplicates
# from unsorted linkedlist
 
 
class Node:
 
    def __init__(self, data):
 
        self.data = data
        self.next = None
 
 
class LinkedList:
 
    def __init__(self):
 
        self.head = None
 
    # Function to print nodes in a
    # given linked list
    def printlist(self):
 
        temp = self.head
 
        while (temp):
            print(temp.data, end=" ")
            temp = temp.next
 
    # Function to remove duplicates from a
    # unsorted linked list
    def removeDuplicates(self, head):
 
        # Base case of empty list or
        # list with only one element
        if self.head is None or self.head.next is None:
            return head
 
        # Hash to store seen values
        hash = set()
 
        current = head
        hash.add(self.head.data)
 
        while current.next is not None:
 
            if current.next.data in hash:
                current.next = current.next.next
            else:
                hash.add(current.next.data)
                current = current.next
 
        return head
 
 
# Driver code
if __name__ == "__main__":
 
    # Creating Empty list
    llist = LinkedList()
    llist.head = Node(10)
    second = Node(12)
    third = Node(11)
    fourth = Node(11)
    fifth = Node(12)
    sixth = Node(11)
    seventh = Node(10)
 
    # Connecting second and third
    llist.head.next = second
    second.next = third
    third.next = fourth
    fourth.next = fifth
    fifth.next = sixth
    sixth.next = seventh
 
    # Printing data
    print("Linked List before removing Duplicates.")
    llist.printlist()
    llist.removeDuplicates(llist.head)
    print("\nLinked List after removing duplicates.")
    llist.printlist()
 
# This code is contributed by rajataro0

C#




// C# program to remove duplicates
// from unsorted linkedlist
using System;
using System.Collections.Generic;
 
class removeDuplicates {
    class node {
        public int val;
        public node next;
 
        public node(int val) { this.val = val; }
    }
 
    // Function to remove duplicates from a
    // unsorted linked list
    static void removeDuplicate(node head)
    {
 
        // Hash to store seen values
        HashSet<int> hs = new HashSet<int>();
 
        // Pick elements one by one
        node current = head;
        node prev = null;
        while (current != null) {
            int curval = current.val;
 
            // If current value is seen before
            if (hs.Contains(curval)) {
                prev.next = current.next;
            }
            else {
                hs.Add(curval);
                prev = current;
            }
            current = current.next;
        }
    }
 
    // Function to print nodes in a
    // given linked list
    static void printList(node head)
    {
        while (head != null) {
            Console.Write(head.val + " ");
            head = head.next;
        }
    }
 
    // Driver code
    public static void Main(String[] args)
    {
 
        // The constructed linked list is:
        // 10->12->11->11->12->11->10
        node start = new node(10);
        start.next = new node(12);
        start.next.next = new node(11);
        start.next.next.next = new node(11);
        start.next.next.next.next = new node(12);
        start.next.next.next.next.next = new node(11);
        start.next.next.next.next.next.next = new node(10);
 
        Console.WriteLine("Linked list before removing "
                          + "duplicates :");
        printList(start);
 
        removeDuplicate(start);
 
        Console.WriteLine("\nLinked list after removing "
                          + "duplicates :");
        printList(start);
    }
}
 
// This code is contributed by amal kumar choubey

Javascript




<script>
 
// JavaScript program to remove duplicates
// from unsorted linkedlist
 
     class node {
 
        constructor(val) {
            this.val = val;
            this.next = null;
        }
    }
 
    /*
     Function to remove duplicates
     from a unsorted linked list
     */
    function removeDuplicate( head) {
        // Hash to store seen values
        var hs =  new Set();
 
        /* Pick elements one by one */
        var current = head;
        var prev = null;
        while (current != null) {
            var curval = current.val;
 
            // If current value is seen before
            if (hs.has(curval)) {
                prev.next = current.next;
            } else {
                hs.add(curval);
                prev = current;
            }
            current = current.next;
        }
 
    }
 
    /* Function to print nodes in a
    given linked list */
    function printList( head) {
        while (head != null) {
            document.write(head.val + " ");
            head = head.next;
        }
    }
 
     
        /*
         * The constructed linked list is:
         10->12->11->11->12->11->10
         */
         start = new node(10);
        start.next = new node(12);
        start.next.next = new node(11);
        start.next.next.next = new node(11);
        start.next.next.next.next = new node(12);
        start.next.next.next.next.next = new node(11);
        start.next.next.next.next.next.next = new node(10);
 
        document.write(
        "Linked list before removing duplicates :<br/>"
        );
        printList(start);
 
        removeDuplicate(start);
 
        document.write(
        "<br/>Linked list after removing duplicates :<br/>"
        );
        printList(start);
 
// This code is contributed by todaysgaurav
 
</script>
Output
Linked list before removing duplicates : 
10 12 11 11 12 11 10 
Linked list after removing duplicates : 
10 12 11 

Thanks to bearwang for suggesting this method.
Time Complexity: O(n) on average (assuming that hash table access time is O(1) on average).  
 

 

–oI4
Please write comments if you find any of the above explanations/algorithms incorrect, or a better ways to solve the same problem.
 

 




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