Program to delete all even nodes from a Singly Linked List

Given a singly linked list containing N nodes, the task is to delete all the even nodes from the list.

Examples:

Input: LL = 1 -> 4 -> 3 -> 18 -> 19
Output: 1 -> 3 -> 19

Input: LL = 5 -> 3 -> 6 -> 8 -> 4 -> 1 -> 2 -> 9
Output: 5 -> 3 -> 1 -> 9



Approach:

  • The idea is to traverse the nodes of the singly linked list one by one and get the pointer of the nodes having even data. Delete those nodes by following the approach used in this post.

Below is the implementation of the above idea:

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// C++ implementation to delete all
// even nodes from the singly linked list
  
#include <bits/stdc++.h>
using namespace std;
  
// Node of the singly linked list
struct Node {
    int data;
    struct Node* next;
};
  
// Function to insert a node at
// the beginning of the singly
// Linked 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;
}
  
// Function to delete a node in a
// singly Linked List.
// head_ref --> Pointer to head
// node pointer.
// key --> Node to be deleted
void deleteNode(struct Node** head_ref,
                int key)
{
    // Store head node
    struct Node *temp = *head_ref,
                *prev;
  
    // If head node itself holds
    // the key to be deleted
    if (temp != NULL
        && temp->data == key) {
        // Changed head
        *head_ref = temp->next;
        // Free old head
        free(temp);
        return;
    }
  
    // 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 memory
    free(temp);
}
  
// Function to delete all the
// even nodes from the
// singly linked list
void deleteEvenNodes(Node** head_ref)
{
    Node* ptr = *head_ref;
    // Node* next;
  
    while (ptr != NULL) {
        // next = ptr->next;
        // If true, delete node 'ptr'
        if (ptr->data % 2 == 0)
            deleteNode(head_ref,
                       ptr->data);
        ptr = ptr->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;
    }
}
  
// Driver code
int main()
{
    // Start with the empty list
    Node* head = NULL;
    push(&head, 19);
    push(&head, 18);
    push(&head, 3);
    push(&head, 4);
    push(&head, 1);
  
    printf("Initial List: ");
    printList(head);
  
    deleteEvenNodes(&head);
  
    printf("\nFinal List: ");
    printList(head);
}

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Output:

Initial List:  1 ->  4 ->  3 ->  18 ->  19 -> 
Final List:  1 ->  3 ->  19 ->

Time Complexity: O(N), where N is the total number of nodes.

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