Delete first occurrence of given Key from a Linked List

Last Updated : 15 May, 2023

Given a linked list and a key to be deleted. The task is to delete the first occurrence of the given key from the linked list.

Note: The list may have duplicates.

Examples:

Input: list = 1->2->3->5->2->10, key = 2
Output: 1->3->5->2->10
Explanation: There are two instances of 2. But as per the question we need to delete the first occurrence only.

Input: list = 3->3->3->3, key = 3
Output: 3->3->3

Approach:

The basic idea is to find the first occurrence of the key by traversing from the head and delete the node. The deletion of a node is explained in detail in Deletion in Linked List.

Follow the steps mentioned below to implement the idea:

Start iterating from the given head of the linked list.
If the data of the current node matches the given key, this is the first occurrence of the given key.
So, delete this node and return the head of the modified linked list.

Below is the implementation of the above approach.

// A complete working C code to demonstrate
// deletion of given key 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 a value, 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) {

        // Changed head
        *head_ref = temp->next;
        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(temp);
}

// 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;
}

C++

// A complete working C++ program to demonstrate
// deletion of given key in singly linked

#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 a value, 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;
}

Java

// A complete working Java program to demonstrate
// deletion of given key in singly linked list

import java.io.*;

public class LinkedList {
    Node head;

    // 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) {

            // 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) {
            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("Created Linked List:");
        llist.printList();

        // Delete node with data 1
        llist.deleteNode(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(temp.data, end =" "),
            temp = temp.next


# Driver code
if __name__ == '__main__':
    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()

// 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("Created Linked List:");
        llist.printList();

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

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

JavaScript

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

    // Head of list
    var head;

    // 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) {
            
            // 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.
    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) {
            console.log(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);

    console.log("Created Linked List: ");
    printList();

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

    console.log("Linked List after Deletion of 1:");
    printList();

Output

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

Time Complexity: O(N) where N is the length of the linked list
Auxiliary Space: O(1)

Delete First Occurrence of Given Element from an Array

RishabhPrabhu

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Delete first occurrence of given Key from a Linked List

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