Delete a Node from linked list without head pointer

You are given a singly linked list and pointer which is pointing to the node which is required to be deleted. Any information about the head pointer or any other node is not given. You need to write a function to delete that node from the linked list. Your function will take only one argument, i.e., a pointer to the node which is to be delete

Note: No head reference is given to you. It is guaranteed that the node to be deleted is not the last node:

A linked list is built as: 

Definition of each node is as follows: 
struct Node {
    int data;
    struct Node* next;
}; 

Examples: 

Consider below LL for given examples:

Input: C (a pointer to C)
Output: A–>B–>D–>E–>F

Input: A (a pointer to A)
Output: B–>D–>E–>F

Approach: To solve the problem follow the below idea:

• Why conventional deletion method would fail here?
  It would be a simple deletion problem from the singly linked list if the head pointer was given because for deletion you must know the previous node and you can easily reach there by traversing from the head pointer. Conventional deletion is impossible without knowledge of the previous node of a node that needs to be deleted. 
• How to delete the node when you dont have the head pointer?
  The trick here is we can copy the data of the next node to the data field of the current node to be deleted. Then we can move one step forward. Now our next has become the current node and the current has become the previous node. Now we can easily delete the current node by conventional deletion methods. 

Illustration of the above approach:

For example, suppose we need to delete B and a pointer to B is given

If we had a pointer to A, we could have deleted B easily. 
But here we will copy the data field of C to the data field of B. Then we will move forward. 

Now we are at C and we have a pointer to B i.e. the previous pointer. So we will delete C. 

That’s how node B will be deleted. 

Below image is a dry run of the above approach:

Below is the implementation of the above approach:

C++

// C++ program to delete a node
#include <bits/stdc++.h>
using namespace std;

/* Link list node */
struct Node {
    int data;
    struct Node* next;
};

// Function to delete the node without head
void deleteNodeWithoutHead(struct Node* pos)
{
    if (pos == NULL) // If linked list is empty
        return;
    else {

        if (pos->next == NULL) {
            printf("This is last node, require head, can't "
                   "be freed\n");
            return;
        }

        struct Node* temp = pos->next;

        // Copy data of the next node to current node
        pos->data = pos->next->data;

        // Perform conventional deletion
        pos->next = pos->next->next;

        free(temp);
    }
}

// Function to print the linked list
void print(Node* head)
{
    Node* temp = head;
    while (temp) {
        cout << temp->data << " -> ";
        temp = temp->next;
    }

    cout << "NULL";
}

void push(struct Node** head_ref, int new_data)
{
    /* allocate node */
    struct Node* new_node = new Node();

    /* put in the data */
    new_node->data = new_data;

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

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

// Driver Code
int main()
{
    /* Start with the empty list */
    struct Node* head = NULL;

    // create linked 35->15->4->20
    push(&head, 20);
    push(&head, 4);
    push(&head, 15);
    push(&head, 35);
    cout << "Initial Linked List: \n";
    print(head);
    cout << endl << endl;

    // Delete 15 without sending head
    Node* del = head->next;

    // Function call
    deleteNodeWithoutHead(del);

    // Print the final linked list
    cout << "Final Linked List after deletion of 15:\n";
    print(head);

    return 0;

    // This code has been contributed by Striver
}

Java

// Java Program to delete a node

class Node{
  int data;
  Node next;
  Node(int val){
    data = val;
    next = null;
  }
}

class LinkedList{
  Node head;

  // Function to delete the node without head
  void deleteNodeWithoutHead(Node pos){
    if(pos == null) // If linked list is empty
      return;
    else{
      if(pos.next == null){
        System.out.println("This is last node, require head, can't be freed\n");
        return;
      }
    }
    // copy data of the next node to current node
    pos.data = pos.next.data;

    // perform conventional deletion
    pos.next = pos.next.next;
  }

  public void push(int new_data){
    // allocate node and put in the data
    Node new_node = new Node(new_data);

    // link the old list of the new node
    new_node.next = head;

    // move the head to point to the new node
    head = new_node;
  }

  public void printList(){
    Node temp = head;
    while(temp != null){
      System.out.print(temp.data + "->");
      temp = temp.next;
    }
    System.out.println("NULL\n");
  }

  public static void main(String args[]){
    LinkedList llist = new LinkedList();

    // create linked list 35->15->4->20
    llist.push(20);
    llist.push(4);
    llist.push(15);
    llist.push(35);

    System.out.println("Initial Linked List : ");
    llist.printList();

    // Delete 15 without sending head
    Node del = llist.head.next;

    // Function call
    llist.deleteNodeWithoutHead(del);

    // Print the final Linked List
    System.out.println("Final Linked List after deletion of 15 : ");
    llist.printList();
  }
}

// This code is contributed by Yash Agarwal(yashagarwal2852002)

Python3

class Node:
    def __init__(self, val):
        self.data = val
        self.next = None

class LinkedList:
    def __init__(self):
        self.head = None

    # Function to delete the node without head
    def delete_node_without_head(self, pos):
        if pos is None:  # If linked list is empty
            return
        elif pos.next is None:
            print("This is last node, require head, can't be freed\n")
            return

        # copy data of the next node to current node
        pos.data = pos.next.data

        # perform conventional deletion
        pos.next = pos.next.next

    def push(self, new_data):
        # allocate node and put in the data
        new_node = Node(new_data)

        # link the old list of the new node
        new_node.next = self.head

        # move the head to point to the new node
        self.head = new_node

    def print_list(self):
        temp = self.head
        while temp is not None:
            print(temp.data, end='->')
            temp = temp.next
        print("NULL\n")

if __name__ == '__main__':
    llist = LinkedList()

    # create linked list 35->15->4->20
    llist.push(20)
    llist.push(4)
    llist.push(15)
    llist.push(35)

    print("Initial Linked List : ")
    llist.print_list()

    # Delete 15 without sending head
    del_node = llist.head.next

    # Function call
    llist.delete_node_without_head(del_node)

    # Print the final Linked List
    print("Final Linked List after deletion of 15 : ")
    llist.print_list()

# This code has been contributed by Mayank Bohra

C#

// C# Program to delete a node

using System;

class Node
{
public int data;
public Node next;
  public Node(int val)
{
    data = val;
    next = null;
}
}

class LinkedList
{
public Node head;
  public LinkedList()
{
    head = null;
}

// Function to delete the node without head
public void delete_node_without_head(Node pos)
{
    if (pos == null)  // If linked list is empty
    {
        return;
    }
    else if (pos.next == null)
    {
        Console.WriteLine("This is last node, require head, can't be freed\n");
        return;
    }

    // copy data of the next node to current node
    pos.data = pos.next.data;

    // perform conventional deletion
    pos.next = pos.next.next;
}

public void push(int new_data)
{
    // allocate node and put in the data
    Node new_node = new Node(new_data);

    // link the old list of the new node
    new_node.next = head;

    // move the head to point to the new node
    head = new_node;
}

public void print_list()
{
    Node temp = head;
    while (temp != null)
    {
        Console.Write(temp.data + "->");
        temp = temp.next;
    }
    Console.WriteLine("NULL\n");
}
}

class Program
{
static void Main(string[] args)
{
LinkedList llist = new LinkedList();
    // create linked list 35->15->4->20
    llist.push(20);
    llist.push(4);
    llist.push(15);
    llist.push(35);

    Console.WriteLine("Initial Linked List : ");
    llist.print_list();

    // Delete 15 without sending head
    Node del_node = llist.head.next;

    // Function call
    llist.delete_node_without_head(del_node);

    // Print the final Linked List
    Console.WriteLine("Final Linked List after deletion of 15 : ");
    llist.print_list();
}
}

//This code is contributed by shivamsharma215

Javascript

class Node{

    constructor(val){
        this.data = val;
        this.next = null;
    }
}

class LinkedList{

    constructor(){
        this.head = null;
    }

    // Function to delete the node without head
    delete_node_without_head(pos){
        if(pos == null) //  If linked list is empty
            return;
        else if(pos.next == null){
            console.log("This is last node, require head, can't be freed\n");
            return;
        }

        // copy data of the next node to current node
        pos.data = pos.next.data;

        // perform conventional deletion
        pos.next = pos.next.next;
    }

    push(new_data){
        // allocate node and put in the data
        let new_node = new Node(new_data);

        // link the old list of the new node
        new_node.next = this.head;

        // move the head to point to the new node
        this.head = new_node;
    }

    print_list(){

        let temp = this.head;
        while(temp != null){
            console.log(temp.data + "->");
            temp = temp.next;
        }
        console.log("NULL \n\n");
    }


}

let llist = new LinkedList();

// create linked list 35->15->4->20
llist.push(20);
llist.push(4);
llist.push(15);
llist.push(35);

console.log("Initial Linked List : \n");
llist.print_list();

// Delete 15 without sending head
let del_node = llist.head.next;

// Function call
llist.delete_node_without_head(del_node);

// Print the final Linked List
console.log("Final Linked List after deletion of 15 : \n");
llist.print_list();

// This code has been contributed by Nidhi goel.

Output

Initial Linked List: 
35 -> 15 -> 4 -> 20 -> NULL

Final Linked List after deletion of 15:
35 -> 4 -> 20 -> NULL

Time complexity: O(1) since performing constant operations and modifying only a single pointer to delete node
Auxiliary Space: O(1)

This article is contributed by Jeet Jain. Please see Given only a pointer to a node to be deleted in a singly linked list, how do you delete it for more details and complete implementation.