public ListNode RemoveNthFromEnd(ListNode head, int n) {ListNode zero=new ListNode(0);zero.next=head;ListNode fast=zero;ListNode slow=zero;//若链表只有一个结点，直接清空if(head.next==null){head=null;return head;}//将快指针放向前移动n次for(int i=1;i<=n+1;i++)fast=fast.next;//如果快指针已经为空，说明清除的是第一个结点，直接将head指向其后继结点if(fast==null){head=head.next;return head;}//循环遍历链表直到快指针到达尾部while(fast!=null){fast=fast.next;slow=slow.next;}//此时slow的后继结点为要删除的倒数第n个结点slow.next=slow.next.next;return zero.next;
}    

public ListNode MergeTwoLists(ListNode l1, ListNode l2) {if(l1==null)return l2;if(l2==null)return l1;ListNode newList=null;//递归调用自身，递归到两链表中最后剩的最大的链表，赋值为前一个小值的nextif(l1.val<=l2.val){newList=l1;newList.next=MergeTwoLists(l1.next,l2);}else{newList=l2;newList.next=MergeTwoLists(l1,l2.next);}return newList;
}

public bool HasCycle(ListNode head) {//快慢指针检测单链表中是否存在环if(head==null||head.next==null)return false;//快指针一次前进n倍慢指针的步数ListNode fastNode=head.next;ListNode slowNode=head;//如果存在环，二者一定会碰上，得到相等的情况while(fastNode!=slowNode){if(fastNode==null||fastNode.next==null)return false;fastNode=fastNode.next.next;slowNode=slowNode.next;}return true;
}

public ListNode ReverseList(ListNode head) {if(head==null)return head;ListNode pre=head;ListNode cur=head.next;ListNode temp;while (cur!= null){//第一次循环时将第二个结点提到第一个节点前，此时链表断开。//之后的循环就是不断将后部分的首结点提到前部分的首位。//同时更新pre、cur和temp指针为前部分的首结点、后部分的首结点和后部分首结点的后继结点。 temp=cur.next;//设置后继结点指针cur.next=pre;//将当前结点提到前驱结点前pre=cur;//设置前驱结点指针cur=temp;//设置当前结点指针
        }head.next=null;return pre;
}

public class MyLinkedList {//单链表结点类public class Node{public int val;public Node next;//实例构造器public Node(int item){val=item;next=null;}public Node(){val=0;next=null;}}//头结点和数量public Node head;public int count;/** Initialize your data structure here. */public MyLinkedList() {head=null;count=0;}/** Get the value of the index-th node in the linked list. If the index is invalid, return -1. */public int Get(int index) {//判断索引是否越界if(index>=count)return -1;Node p=head;int num=0;//遍历链表，找到索引指向的结点位置，返回该结点的值while(true){if(num==index)return p.val;num++;p=p.next;}}/** Add a node of value val before the first element of the linked list. After the insertion, the new node will be the first node of the linked list. */public void AddAtHead(int val) {//更新headNode p=new Node(val);p.next=head;head=p;count++;}/** Append a node of value val to the last element of the linked list. */public void AddAtTail(int val) {Node p=new Node(val);Node q=head;//遍历链表找到尾部，添加节点while(q.next!=null){q=q.next;}q.next=p;count++;}/** Add a node of value val before the index-th node in the linked list. If index equals to the length of linked list, the node will be appended to the end of linked list. If index is greater than the length, the node will not be inserted. */public void AddAtIndex(int index, int val) {//索引为0直接执行在头结点前添加结点的方法if(index==0){AddAtHead(val);return;}//索引和链表长度相直接执行在尾部添加结点的方法if(index==count){AddAtTail(val);return;}//索引越界直接返回if(index>=count)return;int num=0;Node p=head;//循环遍历，在索引指向结点之前添加结点while(true){num++;if(num==index){Node q=new Node(val);q.next=p.next;p.next=q;count++;return;}p=p.next;}}/** Delete the index-th node in the linked list, if the index is valid. */public void DeleteAtIndex(int index) {//索引越界直接返回if(index>=count)return;int num=0;Node p=head;//遍历链表，删除索引指向的结点while(true){num++;if(num==index){p.next=p.next.next;count--;return;}p=p.next;}}
}/*** Your MyLinkedList object will be instantiated and called as such:* MyLinkedList obj = new MyLinkedList();* int param_1 = obj.Get(index);* obj.AddAtHead(val);* obj.AddAtTail(val);* obj.AddAtIndex(index,val);* obj.DeleteAtIndex(index);*/

public ListNode MiddleNode(ListNode head) {ListNode zero = new ListNode(0);zero = head;//快指针和慢指针以head为next开始ListNode fast = zero;ListNode slow = zero;if (head.next == null)return head;while (true){//快慢指针步进fast = fast.next.next;slow = slow.next;//直到快指针为null或快指针的后继结点为null，返回慢指针指向的结点if (fast == null||fast.next == null)return slow;}
}