Delete all occurrences of a given key in a linked list

Given a singly linked list, delete all occurrences of a given key in it. For example, consider the following list.

Input: 2 -> 2 -> 1 -> 8 -> 2 ->  3 ->  2 -> 7
       Key to delete = 2
Output:  1 -> 8 -> 3 -> 7 

This is mainly an extension of this post which deletes first occurrence of a given key.

We need to first check for all occurrences at head node and change the head node appropriately. Then we need to check for all occurrences inside a loop and delete them one by one. Following is C implementation for the same.

// C Program to delete all occurrences of a given key in linked list
#include <stdio.h>
#include <stdlib.h>
  
// A linked list node
struct Node
{
    int data;
    struct Node *next;
};
  
/* Given a reference (pointer to pointer) to the head of a list
   and an int, inserts a new node on the front of the list. */
void push(struct Node** head_ref, int new_data)
{
    struct Node* new_node = (struct Node*) malloc(sizeof(struct Node));
    new_node->data  = new_data;
    new_node->next = (*head_ref);
    (*head_ref)    = new_node;
}
  
/* Given a reference (pointer to pointer) to the head of a list and
   a key, deletes all occurrence of the given key in linked list */
void deleteKey(struct Node **head_ref, int key)
{
    // Store head node
    struct Node* temp = *head_ref, *prev;
  
    // If head node itself holds the key or multiple occurrences of key
    while (temp != NULL && temp->data == key)
    {
        *head_ref = temp->next;   // Changed head
        free(temp);               // free old head
        temp = *head_ref;         // Change Temp
    }
  
    // Delete occurrences other than head
    while (temp != NULL)
    {
        // Search for the key to be deleted, keep track of the
        // previous node as we need to change 'prev->next'
        while (temp != NULL && temp->data != key)
        {
            prev = temp;
            temp = temp->next;
        }
  
        // If key was not present in linked list
        if (temp == NULL) return;
  
        // Unlink the node from linked list
        prev->next = temp->next;
  
        free(temp);  // Free memory
  
        //Update Temp for next iteration of outer loop
        temp = prev->next;
    }
}
  
// This function prints contents of linked list starting from
// the given node
void printList(struct Node *node)
{
    while (node != NULL)
    {
        printf(" %d ", node->data);
        node = node->next;
    }
}
  
/* Drier program to test above functions*/
int main()
{
    /* Start with the empty list */
    struct Node* head = NULL;
  
    push(&head, 7);
    push(&head, 2);
    push(&head, 3);
    push(&head, 2);
    push(&head, 8);
    push(&head, 1);
    push(&head, 2);
    push(&head, 2);
  
    int key = 2; // key to delete
  
    puts("Created Linked List: ");
    printList(head);
  
    deleteKey(&head, key);
    puts("\nLinked List after Deletion of 1: ");
  
    printList(head);
    return 0;
}

Output:

Created Linked List:
 2  2  1  8  2  3  2  7
Linked List after Deletion of 1:
 1  8  3  7 

This article is contributed by Saransh. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.



My Personal Notes arrow_drop_up

Article Tags :  
Practice Tags :



Be the First to upvote.


Please write to us at contribute@geeksforgeeks.org to report any issue with the above content.