a_135_Queue_LL.java

public class a_135_Queue_LL {
    static class Node{
        int data ; 
        Node next ; 

        Node(int data){
            this.data = data ;
            this.next = null ;
        }
    }

    static class Queue {
          static Node head = null  ;
          static Node tail = null  ;

   
   // methods 
    public static boolean isEmpty(){
       return head == null && tail == null ;
   } 

   // add 
   public static void add(int data) {
       Node newNode = new Node(data) ;
   
    // add first element 
    if(head == null) {
        head = tail = newNode ;
        return ;
    }

      tail.next = newNode ;
      tail = newNode ;
   
   }

   // Remove 
   public static int remove(){
    if(isEmpty()){
        System.out.println("Queue is Empty");
        return -1; 
    }

    int front = head.data ;

    // single element 
    if(head == tail){
        head = tail = null ;
    }
    else{
        head = head.next ;
    }
    
    return front ;
   }

   // peek
   public static int peek(){
    if(isEmpty()){
        System.out.println("Queue is Empty");
        return -1; 
    }

    return head.data ;
   }

}

public static void main(String[] args) {
    Queue q = new Queue()  ;
 
    q.add(1); 
    q.add(2); 
    q.add(3);

    System.out.println(q.remove());
    q.add(4);
    System.out.println(q.remove());
    q.add(5);
    

    while(!q.isEmpty()){
        System.out.println(q.peek());
        q.remove() ;
    }
    
     
}

}