Remove duplicates from a sorted doubly linked list

3

Given a sorted doubly linked list containing n nodes. The problem is to remove duplicate nodes from the given list.

Examples:

Algorithm:

removeDuplicates(head_ref, x)
      if head_ref == NULL
          return
      Initialize current = head_ref
      while current->next != NULL
           if current->data == current->next->data
               deleteNode(head_ref, current->next)
           else
               current = current->next

The algorithm for deleteNode(head_ref, current) (which deletes the node using the pointer to the node) is discussed in this post.

/* C++ implementation to remove duplicates from a 
   sorted doubly linked list */
#include <bits/stdc++.h>

using namespace std;

/* a node of the doubly linked list */
struct Node {
    int data;
    struct Node* next;
    struct Node* prev;
};

/* Function to delete a node in a Doubly Linked List.
   head_ref --> pointer to head node pointer.
   del  -->  pointer to node to be deleted. */
void deleteNode(struct Node** head_ref, struct Node* del)
{
    /* base case */
    if (*head_ref == NULL || del == NULL)
        return;

    /* If node to be deleted is head node */
    if (*head_ref == del)
        *head_ref = del->next;

    /* Change next only if node to be deleted 
        is NOT the last node */
    if (del->next != NULL)
        del->next->prev = del->prev;

    /* Change prev only if node to be deleted 
       is NOT the first node */
    if (del->prev != NULL)
        del->prev->next = del->next;

    /* Finally, free the memory occupied by del*/
    free(del);
}

/* function to remove duplicates from a 
   sorted doubly linked list */
void removeDuplicates(struct Node** head_ref)
{
    /* if list is empty */
    if ((*head_ref) == NULL)
        return;

    struct Node* current = *head_ref;
    struct Node* next;

    /* traverse the list till the last node */
    while (current->next != NULL) {

        /* Compare current node with next node */
        if (current->data == current->next->data)

            /* delete the node pointed to by
              'current->next' */
            deleteNode(head_ref, current->next);

        /* else simply move to the next node */
        else
            current = current->next;
    }
}

/* Function to insert a node at the beginning 
   of the Doubly Linked 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;

    /* since we are adding at the begining,
    prev is always NULL */
    new_node->prev = NULL;

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

    /* change prev of head node to new node */
    if ((*head_ref) != NULL)
        (*head_ref)->prev = new_node;

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

/* Function to print nodes in a given doubly linked list */
void printList(struct Node* head)
{
    /* if list is empty */
    if (head == NULL)
        cout << "Doubly Linked list empty";

    while (head != NULL) {
        cout << head->data << " ";
        head = head->next;
    }
}

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

    /* Create the doubly linked list:
     4<->4<->4<->4<->6<->8<->8<->10<->12<->12 */
    push(&head, 12);
    push(&head, 12);
    push(&head, 10);
    push(&head, 8);
    push(&head, 8);
    push(&head, 6);
    push(&head, 4);
    push(&head, 4);
    push(&head, 4);
    push(&head, 4);

    cout << "Original Doubly linked list:n";
    printList(head);

    /* remove duplicate nodes */
    removeDuplicates(&head);

    cout << "\nDoubly linked list after"
             " removing duplicates:n";
    printList(head);

    return 0;
}

Output:

Original Doubly linked list:
4 4 4 4 6 8 8 10 12 12
Doubly linked list after removing duplicates:
4 6 8 10 12

Time Complexity: O(n)

This article is contributed by Ayush Jauhari. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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