Explain the difference between a singly-linked list and a doubly-linked list.?

Both singly-linked lists and doubly-linked lists are linear data structures that store a collection of elements. However, the key difference between them is the number of links between each node and its neighboring nodes.

A singly-linked list is a linear data structure in which each node contains a value and a single link pointing to the next node in the list. The last node in the list has a link pointing to null, indicating the end of the list. Here is an example of a singly-linked list:

class Node {
    int value;
    Node next;

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

class SinglyLinkedList {
    Node head;

    public SinglyLinkedList() {
        this.head = null;
    }

    public void add(int value) {
        Node newNode = new Node(value);
        if (head == null) {
            head = newNode;
        } else {
            Node current = head;
            while (current.next != null) {
                current = current.next;
            }
            current.next = newNode;
        }
    }
}

In this example, the Node class represents a node in the singly-linked list, with an integer value and a single link pointing to the next node in the list. The SinglyLinkedList class represents the singly-linked list itself, with a head node and a method for adding nodes to the end of the list.

A doubly-linked list, on the other hand, is a linear data structure in which each node contains a value and two links, one pointing to the previous node and one pointing to the next node in the list. The first node in the list has a link pointing to null for the previous node, and the last node in the list has a link pointing to null for the next node. Here is an example of a doubly-linked list:

class Node {
    int value;
    Node prev;
    Node next;

    public Node(int value) {
        this.value = value;
        this.prev = null;
        this.next = null;
    }
}

class DoublyLinkedList {
    Node head;
    Node tail;

    public DoublyLinkedList() {
        this.head = null;
        this.tail = null;
    }

    public void add(int value) {
        Node newNode = new Node(value);
        if (head == null) {
            head = newNode;
            tail = newNode;
        } else {
            newNode.prev = tail;
            tail.next = newNode;
            tail = newNode;
        }
    }
}

In this example, the Node class represents a node in the doubly-linked list, with an integer value and two links pointing to the previous node and the next node in the list. The DoublyLinkedList class represents the doubly-linked list itself, with a head node, a tail node, and a method for adding nodes to the end of the list.

In summary, the main difference between a singly-linked list and a doubly-linked list is that a singly-linked list has a single link pointing to the next node, while a doubly-linked list has two links pointing to the previous node and the next node. A doubly-linked list provides more flexibility in terms of traversing the list in both forward and backward directions, but requires more memory to store the extra links.