How does the internal structure of a doubly linked list differ from a singly linked list?

In a singly linked list, each element (node) contains data and a reference only to the next element of the list. This allows traversal of the list in only one direction — from the beginning to the end.

In a doubly linked list, each node contains three parts: data, a reference to the next element, and a reference to the previous element. This enables movement through the list in both directions — forward and backward.

Example of a doubly linked list node structure in Java:

class Node {
    int data;
    Node next; // reference to the next node
    Node prev; // reference to the previous node

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

Thus, the main difference is the presence of an additional reference to the previous element in a doubly linked list, which increases navigation flexibility but requires more memory for storing references.