Java - Delete first node by key of the Circular Singly Linked List

In this method, first node in the circular singly 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 list becomes 10->30->10->20.

First, the head of the linked list is checked for null value. If the head is not null, the value stored in it is equal to the key, and head is the only element in the list then make the head as null. If the head is not null, the value stored in it is equal to the key, the list contains more than one element then make head next as head and update the link of last element of the list with the new 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 4-step process.

void pop_first(int key) {
  
  //1. if head is not null, create two nodes- temp
  //   and nodeToDelete - to traverse and track 
  //   the node to delete  
  if(this.head != null) {
    Node temp = this.head;
    Node nodeToDelete = this.head;
    
    //2. if the value store at head is the key and head
    //   is the only element in the list, make it null
    if(temp.data == key) {
      if(temp.next == this.head) {
        this.head = null;
      } else {

        //3. if the value store at head is the key and list
        //   contains more than 1 elements, traverse to the
        //   last element of the list and link it to the new head
        while(temp.next != this.head) {
          temp = temp.next;
        }
        this.head = this.head.next;
        temp.next = this.head; 
        nodeToDelete = null; 
      }
    } else {
      
      //4. Else, traverse to the node previous to the 
      //   node with value equal to key, and adjust links 
      while(temp.next != this.head) {
        if(temp.next.data == key) {
          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 circular singly 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;
      newNode.next = head;
    } else {
      Node temp = new Node();
      temp = head;
      while(temp.next != head)
        temp = temp.next;
      temp.next = newNode;
      newNode.next = head;
    }    
  }

  //Delete first node by key
  void pop_first(int key) {
    if(this.head != null) {
      Node temp = this.head;
      Node nodeToDelete = this.head;
      if(temp.data == key) {
        if(temp.next == this.head) {
          this.head = null;
        } else {
          while(temp.next != this.head) {
            temp = temp.next;
          }
	      this.head = this.head.next;
          temp.next = this.head; 
          nodeToDelete = null; 
        }
      } else {
        while(temp.next != this.head) {
          if(temp.next.data == key) {
            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(true) {
        System.out.print(temp.data + " ");
        temp = temp.next;
        if(temp == this.head)
          break;
      }
      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 in 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