java - 在单向链表中添加双向链表节点

Question

给定单链表:1 -> 2 -> 3 -> 4 -> 5 -> null
修改中间元素为双向链表节点
这里的中间元素是 3
3 -> next 应该指向 4
3 -> prev 应该指向 1

任何人都可以建议如何做到这一点？面试官给了我提示使用界面。但我无法弄清楚如何。
我必须遍历这个链表并打印所有节点，当它到达中间时，打印 next 和 prev 指向的位置，然后打印到列表的末尾。
预期输出:1, 2, Middle: 3, Prev: 1, Next: 4, 5

我在添加中间节点时遇到问题。

Answer 1

所以，这个“有效”，但如果希望在面试中回答这个问题，那就太麻烦了。

链表

public class LinkedList {

    public interface Linkable<V, L extends Linkable> {
        V getValue();
        L getNext();
        void setNext(L link);
    }

    public static class Node implements Linkable<Integer, Linkable> {
        int value;
        Linkable next;

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

        @Override
        public Integer getValue() {
            return value;
        }

        @Override
        public Linkable getNext() {
            return next;
        }

        @Override
        public void setNext(Linkable link) {
            this.next = link;
        }
    }

    private Linkable head;

    public boolean isEmpty() {
        return this.head == null;
    }

    public Linkable getHead() {
        return head;
    }

    public void add(int v) {
        Node next = new Node(v);
        if (isEmpty()) {
            this.head = next;
        } else {
            Linkable tmp = this.head;
            while (tmp.getNext() != null) {
                tmp = tmp.getNext();
            }
            tmp.setNext(next);
        }
    }
}

---

界面

interface DoublyLinkable<V, L extends LinkedList.Linkable> extends LinkedList.Linkable<V,L> {
    LinkedList.Linkable getPrev();
    void setPrev(LinkedList.Linkable prev);
}

双节点

public class DoubleNode extends LinkedList.Node implements DoublyLinkable<Integer, LinkedList.Linkable> {
    LinkedList.Linkable prev;

    public DoubleNode(int value) {
        super(value);
    }

    @Override
    public LinkedList.Linkable getPrev() {
        return prev;
    }

    @Override
    public void setPrev(LinkedList.Linkable prev) {
         this.prev = prev;
    }
}

---

司机

输出

1, 2, Middle: 3, Prev: 1, Next: 4, 5

---

public class Driver {

    public static LinkedList getList() {
        LinkedList list = new LinkedList();
        for (int i = 1; i <= 5; i++) {
            list.add(i);
        }
        return list;
    }

    public static void main(String[] args) {

        LinkedList list = getList();

        LinkedList.Linkable head = list.getHead();
        LinkedList.Linkable beforeMiddle = null;
        LinkedList.Linkable middle = list.getHead();
        LinkedList.Linkable end = list.getHead();

        if (head != null) {
            // find the middle of the list
            while (true) {
                if (end.getNext() == null || end.getNext().getNext() == null) break;

                beforeMiddle = middle;
                middle = middle.getNext();

                end = end.getNext().getNext();
            }

            // Replace middle by reassigning the pointer to it
            if (beforeMiddle != null) {

                DoubleNode n = new DoubleNode((int) middle.getValue()); // same value
                n.setPrev(list.getHead()); // point back to the front
                n.setNext(middle.getNext()); // point forward to original value

                beforeMiddle.setNext((DoublyLinkable) n);
                middle = beforeMiddle.getNext();
            }

            // Build the "expected" output
            StringBuilder sb = new StringBuilder();
            final String DELIMITER = ", ";
            head = list.getHead();
            boolean atMiddle = false;
            if (head != null) {
                do {
                    if (head instanceof DoublyLinkable) {
                        atMiddle = true;
                        String out = String.format("Middle: %d, Prev: %d, ", (int) head.getValue(), (int) ((DoublyLinkable) head).getPrev().getValue());
                        sb.append(out);
                    } else {
                        if (atMiddle) {
                            sb.append("Next: ");
                            atMiddle = false;
                        }
                        sb.append(head.getValue()).append(DELIMITER);
                    }

                    head = head.getNext();
                } while (head != null);
            }
            sb.setLength(sb.length() - DELIMITER.length());

            System.out.println(sb.toString());
        }

    }
}