GATE-CS-2009 Question 9

Problem Description

Design a program to implement a generic LinkedList class in Java. Your LinkedList class should have the following methods:

• public void insertFront(T data): Inserts an element at the front of the list.
• public void insertBack(T data): Inserts an element at the back of the list.
• public void deleteFront(): Deletes the element at the front of the list.
• public void deleteBack(): Deletes the element at the back of the list
• public void printList(): Prints the elements of the list.

Solution

Here is the implementation of the LinkedList class in Java:

public class LinkedList<T> {
    private Node<T> head;
    private Node<T> tail;

    private class Node<T> {
        private T data;
        private Node<T> next;

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

    public void insertFront(T data) {
        Node<T> newNode = new Node<>(data);
        if (head == null) {
            head = newNode;
            tail = newNode;
        } else {
            newNode.next = head;
            head = newNode;
        }
    }

    public void insertBack(T data) {
        Node<T> newNode = new Node<>(data);
        if (tail == null) {
            head = newNode;
            tail = newNode;
        } else {
            tail.next = newNode;
            tail = newNode;
        }
    }

    public void deleteFront() {
        if (head == null) {
            return;
        }
        if (head == tail) {
            head = null;
            tail = null;
        } else {
            head = head.next;
        }
    }

    public void deleteBack() {
        if (tail == null) {
            return;
        }
        if (head == tail) {
            head = null;
            tail = null;
        } else {
            Node<T> curr = head;
            while (curr.next != tail) {
                curr = curr.next;
            }
            curr.next = null;
            tail = curr;
        }
    }

    public void printList() {
        Node<T> curr = head;
        while (curr != null) {
            System.out.print(curr.data + " ");
            curr = curr.next;
        }
        System.out.println();
    }
}

Explanation

The LinkedList class uses a private inner class Node to construct a node containing the data and a reference to the next node in the list. The LinkedList class has instance variables head and tail which keep track of the first and last nodes in the list, respectively.

The insertFront method inserts a new node at the front of the list. If the list is empty, head and tail both point to the new node. Otherwise, the new node becomes the new head node and its next reference points to the previous head node.

The insertBack method inserts a new node at the end of the list. If the list is empty, head and tail both point to the new node. Otherwise, the previous tail node's next reference points to the new node and the new node becomes the new tail node.

The deleteFront method removes the first node in the list. If the list is empty, nothing happens. If the list has only one node, head and tail are set to null, indicating an empty list. Otherwise, head is set to the next node in the list.

The deleteBack method removes the last node in the list. If the list is empty, nothing happens. If the list has only one node, head and tail are set to null. Otherwise, it traverses the list to find the second-to-last node, sets its next reference to null and sets that node as the new tail.

The printList method traverses the list and prints out each element followed by a space. It then prints a new line.

Conclusion

The LinkedList class is a fundamental data structure in computer science. This implementation provides a basic, generic implementation of a singly linked list in Java. It can be used in many different applications, including big data processing, machine learning, and web development.