Delete all occurrences of a given key in a doubly linked list

Given a doubly linked list and a key x. The problem is to delete all occurrences of the given key x from the doubly linked list.

Examples:

Algorithm:

delAllOccurOfGivenKey(head_ref, x)
      if head_ref == NULL
          return
      Initialize current = head_ref
      Declare next
      while current != NULL
           if current->data == x
               next = current->next
               deleteNode(head_ref, current)
               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 delete all occurrences 
   of a given key in a 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 delete all occurrences of the given
    key 'x' */
void deleteAllOccurOfX(struct Node** head_ref, int x)
{
    /* if list is empty */
    if ((*head_ref) == NULL)
        return;

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

    /* traverse the list up to the end */
    while (current != NULL) {

        /* if node found with the value 'x' */
        if (current->data == x) {

            /* save current's next node in the 
               pointer 'next' */
            next = current->next;

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

            /* update current */
            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:
           2<->2<->10<->8<->4<->2<->5<->2 */
    push(&head, 2);
    push(&head, 5);
    push(&head, 2);
    push(&head, 4);
    push(&head, 8);
    push(&head, 10);
    push(&head, 2);
    push(&head, 2);

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

    int x = 2;

    /* delete all occurrences of 'x' */
    deleteAllOccurOfX(&head, x);

    cout << "\nDoubly linked list after deletion of "
         << x << ":n";
    printList(head);

    return 0;
}

Output:

Original Doubly linked list:
2 2 10 8 4 2 5 2
Doubly linked list after deletion of 2:
10 8 4 5

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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