How to determine the presence of doubly linked connections in a list of elements?

A doubly linked list is a data structure where each element (node) contains references to the next and previous elements. To determine if a list of elements has doubly links, you need to check that each node has two references: to the next and to the previous element.

In Java, if you have a node class, for example:

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

then the presence of the prev field indicates doubly linked. If only the next field exists, then the list is singly linked.

If you have a list object, you can traverse it and check that for each node node.next.prev == node (if node.next is not null). This guarantees the correctness of the doubly links.

Example check:

boolean isDoublyLinked(Node head) {
    Node current = head;
    while (current != null && current.next != null) {
        if (current.next.prev != current) {
            return false;
        }
        current = current.next;
    }
    return true;
}