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Java - Delete first node by key of the Linked List



In this method, first node in the linked list with specified key (value) is deleted. For example - if the given List is 10->20->30->10->20 and the first occurrence of 20 is deleted, the Linked List becomes 10->30->10->20.

First, the head of the linked list is checked for null value. If the head is not null and the value stored in it is equal to the key, make head next as head and delete previous head. Else, traverse to the node previous to the node with value equal to key, and adjust links.

The function pop_first is created for this purpose. It is a 3-step process.

void pop_first(int key) {     
  Node temp = head;
  //1. check if the head is not null
  if(temp != null) {
    
    //2. if head is not null and value stored at head
    //   is equal to the key, make head next as head
    //   and delete previous head
    if(temp.data == key) {
      Node nodeToDelete = head;
      head = head.next;
      nodeToDelete = null;
    } else {

      //3. Else, traverse to the node previous to the 
      //   node with value equal to key, and adjust links 
      while(temp.next != null) {
        if(temp.next.data == key) {
          Node nodeToDelete = temp.next;
          temp.next = temp.next.next;
          nodeToDelete = null;
          break; 
        }
        temp = temp.next;
      }
    }
  }
} 

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

//node structure
class Node {
    int data;
    Node next;
};

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;
    if(head == null) {
      head = newNode;
    } else {
      Node temp = new Node();
      temp = head;
      while(temp.next != null)
        temp = temp.next;
      temp.next = newNode;
    }    
  }

  //Delete first node by key
  void pop_first(int key) {     
    Node temp = head;
    if(temp != null) {
      if(temp.data == key) {
        Node nodeToDelete = head;
        head = head.next;
        nodeToDelete = null;
      } else {
        while(temp.next != null) {
          if(temp.next.data == key) {
            Node nodeToDelete = temp.next;
            temp.next = temp.next.next;
            nodeToDelete = null;
            break; 
          }
          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 first occurrence of 20
    MyList.pop_first(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 20 

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