public class LinkedList<Item> {private Node first;private class Node {Item data;Node next;}...}

public void insert(Item item) {Node oldFirst = first;first = new Node();first.item = item;first.next = oldFirst;itemCount++;
}

public Item delete() {if (first != null) {Item item = first.item;first = first.next;return item;} else {throw new NullPointerException("There's no Node in the linked list.");
}

public class DoubleLinkedList<Item> {private Node first;private Node last;private int itemCount;private class Node {Node prev;Node next;Item item;}public void addFirst(Item item) {Node oldFirst = first;first = new Node();first.item = item;first.next = oldFirst;if (oldFirst != null) {oldFirst.prev = first;}if (itemCount == 0) {last = first;}itemCount++;}public void addLast(Item item) {Node oldLast = last;last = new Node();last.item = item;last.prev = oldLast;if (oldLast != null) {oldLast.next = last;}if (itemCount == 0) {first = last;}itemCount++;}public Item delFirst() {if (first == null) {throw new NullPointerException("No node in linked list.");}Item result = first.item;first = first.next;if (first != null) {first.prev = null;}if (itemCount == 1) {last = null;}itemCount--;return result;}public Item delLast() {if (last == null) {throw new NullPointerException("No node in linked list.");}Item result = last.item;last = last.prev;if (last != null) {last.next = null;}if (itemCount == 1) {first = null;}itemCount--;return result;}public void addBefore(Item targetItem, Item item) {//从头开始遍历寻找目标节点Node target = first;if (target == null) {throw new NullPointerException("No node in linked list");}while (target != null && target.item != targetItem) {//继续向后寻找目标节点target = target.next;}if (target == null) {throw new NullPointerException("Can't find target node.");}//现在target为指向目标结点的引用if (target.prev == null) {//此时相当于在表头插入结点
            addFirst(item);} else {Node oldPrev = target.prev;Node newNode = new Node();newNode.item = item;target.prev = newNode;newNode.next = target;newNode.prev = oldPrev;oldPrev.next = newNode;itemCount++;}}public void addAfter(Item targetItem, Item item) {Node target = first;if (target == null) {throw new NullPointerException("No node in linked list.");}while (target != null && target.item != targetItem) {target = target.next;}if (target == null) {throw new NullPointerException("Can't find target node.");}if (target.next == null) {addLast(item);} else {Node oldNext = target.next;Node newNode = new Node();newNode.item = item;target.next = newNode;newNode.prev = target;newNode.next= oldNext;oldNext.prev = newNode;itemCount++;}}}

/*
public class ListNode {int val;ListNode next = null;ListNode(int val) {this.val = val;}
}*/
public class Solution {public ListNode FindKthToTail(ListNode head,int k) {}
}

public class Solution {public ListNode FindKthToTail(ListNode head,int k) {//先求得链表的尺寸，赋值给sizeint size = 0;ListNode current = head;while (current != null) {size++;current = current.next;}//获取next实例域size-k次，即可得到倒数第k个结点（从1开始计数）for (int i = 0; i < size - k; i++) {head = head.next;}return head;}
}

public ListNode ReverseList(ListNode head) {if (head == null) {return null;}ListNode current = head;//原head的next node为nullListNode prevNode = null;ListNode newHead = null;while (current != null) {ListNode nextNode = current.next;current.next = prevNode;if (nextNode == null) {newHead = current;}prevNode = current;current = nextNode;}return newHead;
}