Deletion in Linked List

We have discussed Linked List Introduction and Linked List Insertion in previous posts on a singly linked list.
Let us formulate the problem statement to understand the deletion process. 

Delete from a Linked List:-

You can delete an element in a list from:

• Beginning
• End
• Middle

1) Delete from Beginning:

Point head to the next node i.e. second node
    temp = head
    head = head->next
    
Make sure to free unused memory
    free(temp); or delete temp;

2) Delete from End:

Point head to the previous element i.e. last second element
    Change next pointer to null
    struct node *end = head;
    struct node *prev = NULL;
    while(end->next)
    {
        prev = end;
        end = end->next;
    }
    prev->next = NULL;
    
Make sure to free unused memory
    free(end); or delete end;

3) Delete from Middle:

Keeps track of pointer before node to delete and pointer to node to delete
    temp = head;
    prev = head;
    for(int i = 0; i < position; i++)
    {
        if(i == 0 && position == 1)
            head = head->next;
            free(temp)
        else
        {
            if (i == position - 1 && temp)
            {
                prev->next = temp->next;
                free(temp);
            }
            else
            {
                prev = temp;
                if(prev == NULL) // position was greater than number of nodes in the list
                    break;
                temp = temp->next; 
            }
        }
    }
    

Iterative Method to delete an element from the linked list:

To delete a node from the linked list, we need to do the following steps:

• Find the previous node of the node to be deleted. 
• Change the next of the previous node. 
• Free memory for the node to be deleted.

Below is the implementation to delete a node from the list at some position:

C

// C code to delete a node from linked list
#include <stdio.h>
#include <stdlib.h>

typedef struct Node {
    int number;
    struct Node* next;
} Node;

void Push(Node** head, int A)
{
    Node* n = malloc(sizeof(Node));
    n->number = A;
    n->next = *head;
    *head = n;
}

void deleteN(Node** head, int position)
{
    Node* temp;
    Node* prev;
    temp = *head;
    prev = *head;
    for (int i = 0; i < position; i++) {
        if (i == 0 && position == 1) {
            *head = (*head)->next;
            free(temp);
        }
        else {
            if (i == position - 1 && temp) {
                prev->next = temp->next;
                free(temp);
            }
            else {
                prev = temp;

                // Position was greater than
                // number of nodes in the list
                if (prev == NULL)
                    break;
                temp = temp->next;
            }
        }
    }
}

void printList(Node* head)
{
    while (head) {
        printf("[%i] [%p]->%p\n", head->number, head,
               head->next);
        head = head->next;
    }
    printf("\n\n");
}

// Drivers code
int main()
{
    Node* list = malloc(sizeof(Node));
    list->next = NULL;
    Push(&list, 1);
    Push(&list, 2);
    Push(&list, 3);

    printList(list);

    // Delete any position from list
    deleteN(&list, 1);
    printList(list);
    return 0;
}

Output

[3] [0x1b212c0]->0x1b212a0
[2] [0x1b212a0]->0x1b21280
[1] [0x1b21280]->0x1b21260
[0] [0x1b21260]->(nil)


[2] [0x1b212a0]->0x1b21280
[1] [0x1b21280]->0x1b21260
[0] [0x1b21260]->(nil)

Time Complexity: O(n)
Auxiliary Space: O(1)

Delete the first node in a linked list where data == key:

Since every node of the linked list is dynamically allocated using malloc() in C, we need to call free() for freeing memory allocated for the node to be deleted.

C++

// A complete working C++ program to
// demonstrate deletion in singly
// linked list with class
#include <bits/stdc++.h>
using namespace std;

// A linked list node
class Node {
public:
    int data;
    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(Node** head_ref, int new_data)
{
    Node* new_node = new 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
// the first occurrence of key in linked list
void deleteNode(Node** head_ref, int key)
{

    // Store head node
    Node* temp = *head_ref;
    Node* prev = NULL;

    // 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
        delete temp;
        return;
    }

    // Else Search for the key to be
    // deleted, keep track of the
    // previous node as we need to
    // change 'prev->next'
    else {
        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
        delete temp;
    }
}

// This function prints contents of
// linked list starting from the
// given node
void printList(Node* node)
{
    while (node != NULL) {
        cout << node->data << " ";
        node = node->next;
    }
}

// Driver code
int main()
{

    // Start with the empty list
    Node* head = NULL;

    // Add elements in linked list
    push(&head, 7);
    push(&head, 1);
    push(&head, 3);
    push(&head, 2);

    puts("Created Linked List: ");
    printList(head);

    deleteNode(&head, 1);
    puts("\nLinked List after Deletion of 1: ");

    printList(head);

    return 0;
}

// This code is contributed by ac121102

C

// A complete working C program
// to demonstrate deletion in
// singly 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 the first occurrence of key in
   linked list */
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) {
        *head_ref = temp->next; // Changed head
        free(temp); // free old head
        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(temp); // Free memory
}

// 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 */
    struct Node* head = NULL;

    push(&head, 7);
    push(&head, 1);
    push(&head, 3);
    push(&head, 2);

    puts("Created Linked List: ");
    printList(head);
    deleteNode(&head, 1);
    puts("\nLinked List after Deletion of 1: ");
    printList(head);
    return 0;
}

Java

// A complete working Java program
// to demonstrate deletion
// in singly linked list
class LinkedList {
    Node head; // head of list

    /* Linked list Node*/
    class Node {
        int data;
        Node next;
        Node(int d)
        {
            data = d;
            next = null;
        }
    }

    /* Given a key, deletes the first
       occurrence of key in
     * linked list */
    void deleteNode(int key)
    {
        // Store head node
        Node temp = head, prev = null;

        // If head node itself holds the key to be deleted
        if (temp != null && temp.data == key) {
            head = temp.next; // Changed head
            return;
        }

        // Search for the key to be deleted, keep track of
        // the previous node as we need to change temp.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;
    }

    /* Inserts a new Node at front of the list. */
    public void push(int new_data)
    {
        Node new_node = new Node(new_data);
        new_node.next = head;
        head = new_node;
    }

    /* This function prints contents of linked list starting
       from the given node */
    public void printList()
    {
        Node tnode = head;
        while (tnode != null) {
            System.out.print(tnode.data + " ");
            tnode = tnode.next;
        }
    }

    /* Driver program to test above functions. Ideally this
    function should be in a separate user class. It is kept
    here to keep code compact */
    public static void main(String[] args)
    {
        LinkedList llist = new LinkedList();

        llist.push(7);
        llist.push(1);
        llist.push(3);
        llist.push(2);

        System.out.println("\nCreated Linked list is:");
        llist.printList();

        llist.deleteNode(1); // Delete node with data 1

        System.out.println(
            "\nLinked List after Deletion of 1:");
        llist.printList();
    }
}

Python3

# A complete working Python3 program to
# demonstrate deletion in singly
# linked list with class

# Node class


class Node:

    # Constructor to initialize the node object
    def __init__(self, data):
        self.data = data
        self.next = None


class LinkedList:

    # Function to initialize head
    def __init__(self):
        self.head = None

    # Function to insert a new node at the beginning
    def push(self, new_data):
        new_node = Node(new_data)
        new_node.next = self.head
        self.head = new_node

    # Given a reference to the head of a list and a key,
    # delete the first occurrence of key in linked list
    def deleteNode(self, key):

        # Store head node
        temp = self.head

        # If head node itself holds the key to be deleted
        if (temp is not None):
            if (temp.data == key):
                self.head = temp.next
                temp = None
                return

        # Search for the key to be deleted, keep track of the
        # previous node as we need to change 'prev.next'
        while(temp is not None):
            if temp.data == key:
                break
            prev = temp
            temp = temp.next

        # if key was not present in linked list
        if(temp == None):
            return

        # Unlink the node from linked list
        prev.next = temp.next

        temp = None

    # Utility function to print the linked LinkedList

    def printList(self):
        temp = self.head
        while(temp):
            print(" %d" % (temp.data)),
            temp = temp.next


# Driver program
llist = LinkedList()
llist.push(7)
llist.push(1)
llist.push(3)
llist.push(2)

print("Created Linked List: ")
llist.printList()
llist.deleteNode(1)
print("\nLinked List after Deletion of 1:")
llist.printList()

# This code is contributed by Nikhil Kumar Singh (nickzuck_007)

C#

// A complete working C# program
// to demonstrate deletion in
// singly linked list
using System;
class GFG {

    // Head of list
    Node head;

    // Linked list Node
    public class Node {
        public int data;
        public Node next;
        public Node(int d)
        {
            data = d;
            next = null;
        }
    }

    // Given a key, deletes the first
    // occurrence of key in linked list
    void deleteNode(int key)
    {
        // Store head node
        Node temp = head, prev = null;

        // If head node itself holds
        // the key to be deleted
        if (temp != null && temp.data == key) {
            // Changed head
            head = temp.next;
            return;
        }

        // Search for the key to be
        // deleted, keep track of the
        // previous node as we need
        // to change temp.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;
    }

    // Inserts a new Node at
    // front of the list.
    public void Push(int new_data)
    {
        Node new_node = new Node(new_data);
        new_node.next = head;
        head = new_node;
    }

    // This function prints contents
    // of linked list starting from
    // the given node
    public void printList()
    {
        Node tnode = head;
        while (tnode != null) {
            Console.Write(tnode.data + " ");
            tnode = tnode.next;
        }
    }

    // Driver code
    public static void Main(String[] args)
    {
        GFG llist = new GFG();

        llist.Push(7);
        llist.Push(1);
        llist.Push(3);
        llist.Push(2);

        Console.WriteLine("\nCreated Linked list is:");
        llist.printList();

        // Delete node with data 1
        llist.deleteNode(1);

        Console.WriteLine(
            "\nLinked List after Deletion of 1:");
        llist.printList();
    }
}

// This code is contributed by Rajput-Ji

Javascript

<script>
// A complete working javascript program
// to demonstrate deletion
// in singly linked list

    var head; // head of list

    /* Linked list Node */
    class Node {
        constructor(val) {
            this.data = val;
            this.next = null;
        }
    }

    /*
     * Given a key, deletes the first occurrence of key in linked list
     */
    function deleteNode(key) {
        // Store head node
        var temp = head, prev = null;

        // If head node itself holds the key to be deleted
        if (temp != null && temp.data == key) {
            head = temp.next; // Changed head
            return;
        }

        // Search for the key to be deleted, keep track of
        // the previous node as we need to change temp.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;
    }

    /* Inserts a new Node at front of the list. */
    function push(new_data) {
        var new_node = new Node(new_data);
        new_node.next = head;
        head = new_node;
    }

    /*
     * This function prints contents of linked list starting from the given node
     */
    function printList() {
         tnode = head;
        while (tnode != null) {
            document.write(tnode.data + " ");
            tnode = tnode.next;
        }
    }

    /*
     * Driver program to test above functions. Ideally this function should be in a
     * separate user class. It is kept here to keep code compact
     */


        push(7);
        push(1);
        push(3);
        push(2);

        document.write("Created Linked list is:<br/>");
        printList();

        deleteNode(1); // Delete node with data 1

        document.write("<br/>Linked List after Deletion of 1:<br/>");
        printList();


// This code is contributed by todaysgaurav
</script>

Output

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

Time Complexity: O(n)
Auxiliary Space: O(1)

Recursive Method to delete a node from linked list:

To delete a node of a linked list recursively we need to do the following steps:

• We pass node* (node pointer) as a reference to the function (as in node* &head)
• Now since the current node pointer is derived from the previous node’s next (which is passed by reference) so now if the value of the current node pointer is changed, the previous next node’s value also gets changed which is the required operation while deleting a node (i.e points previous node’s next to current node’s (containing key) next).
• Find the node containing the given value.
• Store this node to deallocate it later using the free() function.
• Change this node pointer so that it points to its next and by performing this previous node’s next also gets properly linked.

Below is the implementation of the above approach.

C++

// C++ program to delete a node in
// singly linked list recursively

#include <bits/stdc++.h>
using namespace std;

struct node {
    int info;
    node* link = NULL;
    node() {}
    node(int a)
        : info(a)
    {
    }
};

// Deletes the node containing 'info'
// part as val and alter the head of
// the linked list (recursive method)
void deleteNode(node*& head, int val)
{

    // Check if list is empty or we
    // reach at the end of the
    // list.
    if (head == NULL) {
        cout << "Element not present in the list\n";
        return;
    }

    // If current node is the
    // node to be deleted
    if (head->info == val) {
        node* t = head;

        // If it's start of the node head
        // node points to second node
        head = head->link;

        // Else changes previous node's
        // link to current node's link
        delete (t);
        return;
    }
    deleteNode(head->link, val);
}

// Utility function to add a
// node in the linked list
// Here we are passing head by
// reference thus no need to
// return it to the main function
void push(node*& head, int data)
{
    node* newNode = new node(data);
    newNode->link = head;
    head = newNode;
}

// Utility function to print
// the linked list (recursive
// method)
void print(node* head)
{

    // cout<<endl gets implicitly
    // typecasted to bool value
    // 'true'
    if (head == NULL and cout << endl)
        return;
    cout << head->info << ' ';
    print(head->link);
}

int main()
{

    // Starting with an empty linked list
    node* head = NULL;

    // Adds new element at the
    // beginning of the list
    push(head, 10);
    push(head, 12);
    push(head, 14);
    push(head, 15);

    // original list
    print(head);

    // Call to delete function
    deleteNode(head, 20);

    // 20 is not present thus no change
    // in the list
    print(head);

    deleteNode(head, 10);
    print(head);

    deleteNode(head, 14);
    print(head);

    return 0;
}

Output

15 14 12 10 
Element not present in the list
15 14 12 10 
15 14 12 
15 12 

Time Complexity: O(n)
Auxiliary Space: O(n) (due to recursion call stack)

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