# Java - Delete all nodes by key of the Doubly Linked List

In this method, all nodes in the doubly linked list with specified key (value) is deleted. For example - if the given list is 10->20->30->10->20 and all occurrences of 20 are deleted, the list becomes 10->30->10.

If the head of the doubly linked list is not null and the value stored at head is equal to key, then adjust the head as head next, delete previous head and adjust links. Keep on doing the same process until the new head becomes null or not equal to the key. After that create a temp node to traverse through the list and delete those nodes with value equal to the key and adjust links accordingly.

The function *pop_all* is created for this purpose. It is a **2-step process**.

void pop_all(int key) { Node nodeToDelete = new Node(); //1. if the head is not null and value stored at head // is equal to key, keep on adjusting the head as // head next, delete previous head and adjust links // until new head becomes null or not equal to key while(head != null && head.data == key) { nodeToDelete = head; head = head.next; nodeToDelete = null; if (head != null) head.prev = null; } //2. create a temp node to traverse through the // list and delete nodes with value equal to // key and adjust links accordingly Node temp = head; if(temp != null) { while(temp.next != null) { if(temp.next.data == key) { nodeToDelete = temp.next; temp.next = temp.next.next; if(temp.next != null) temp.next.prev = temp; nodeToDelete = null; } else { temp = temp.next; } } } }

The below is a complete program that uses above discussed concept to delete all occurrences of the specified key (if exists) of the doubly linked list.

//node structure class Node { int data; Node next; Node prev; }; class LinkedList { Node head; LinkedList(){ head = null; } //Add new element at the end of the list void push_back(int newElement) { Node newNode = new Node(); newNode.data = newElement; newNode.next = null; newNode.prev = null; if(head == null) { head = newNode; } else { Node temp = new Node(); temp = head; while(temp.next != null) temp = temp.next; temp.next = newNode; newNode.prev = temp; } } //Delete all nodes by key void pop_all(int key) { Node nodeToDelete = new Node(); while(head != null && head.data == key) { nodeToDelete = head; head = head.next; nodeToDelete = null; if (head != null) head.prev = null; } Node temp = head; if(temp != null) { while(temp.next != null) { if(temp.next.data == key) { nodeToDelete = temp.next; temp.next = temp.next.next; if(temp.next != null) temp.next.prev = temp; nodeToDelete = null; } else { temp = temp.next; } } } } //display the content of the list void PrintList() { Node temp = new Node(); temp = this.head; if(temp != null) { System.out.print("The list contains: "); while(temp != null) { System.out.print(temp.data + " "); temp = temp.next; } System.out.println(); } else { System.out.println("The list is empty."); } } }; // test the code public class Implementation { public static void main(String[] args) { LinkedList MyList = new LinkedList(); //Add five elements at the end of the list. MyList.push_back(10); MyList.push_back(20); MyList.push_back(30); MyList.push_back(10); MyList.push_back(20); MyList.PrintList(); //Delete all occurrences of 20 MyList.pop_all(20); MyList.PrintList(); } }

The above code will give the following output:

The list contains: 10 20 30 10 20 The list contains: 10 30 10