public class Node {    private String data;    private Node next;
    public Node(String data, Node next) {        this.data = data;        this.next = next;    }
    public String getData() {        return data;    }
    public Node next() {        return next;    }
    public static void main(String[] args) {        Node z = new Node("z", null);        Node y = new Node("y", z);        Node x = new Node("x", y);        Node b = new Node("b", x);        Node a = new Node("a", b);
        Node f = new Node("f", x);        Node e = new Node("e", f);        Node d = new Node("d", e);

        String firstCommonValue = Optional.ofNullable(findFirstCommonNode(a, d))                .map(Node::getData)                .orElse("");        System.out.println("第一个公共元素：" + firstCommonValue);    }
    private static Node findFirstCommonNode(Node head1, Node head2) {        Set<Node> nodesOfList1 = new HashSet<>();        Set<Node> nodesOfList2 = new HashSet<>();        Node tmp = head1;        while (tmp != null && !nodesOfList1.contains(tmp)) {            nodesOfList1.add(tmp);            tmp = tmp.next();        }
        tmp = head2;        while (tmp != null && !nodesOfList2.contains(tmp)) {            if (nodesOfList1.contains(tmp)) {                return tmp;            }            nodesOfList2.add(tmp);            tmp = tmp.next();        }        return null;    }}