Sep 18, 2022 · Sep 13, 2022 · Sep 13, 2022 · Sep 13, 2022 · Sep 18, 2022
diff --git a/problems/0707.设计链表.md b/problems/0707.设计链表.md
Original file line number	Diff line number	Diff line change
Expand Up		@@ -358,100 +358,109 @@ class MyLinkedList {
		return;
		}
		ListNode pred = head;
	for (int i = 0; i < index - 1; i++) {
	for (int i = 0; i < index ; i++) {
		pred = pred.next;
		}
		pred.next = pred.next.next;
		}
		}

		//双链表
	class MyLinkedList {
	class ListNode {
	int val;
	ListNode next,prev;
	ListNode(int x) {val = x;}
	class ListNode{
	int val;
	ListNode next,prev;
	ListNode() {};
	ListNode(int val){
	this.val = val;
		}
	}

	int size;
	ListNode head,tail;//Sentinel node

	/** Initialize your data structure here. */
	class MyLinkedList {

	//记录链表中元素的数量
	int size;
	//记录链表的虚拟头结点和尾结点
	ListNode head,tail;

		public MyLinkedList() {
	size = 0;
	head = new ListNode(0);
	tail = new ListNode(0);
	head.next = tail;
	tail.prev = head;
	//初始化操作
	this.size = 0;
	this.head = new ListNode(0);
	this.tail = new ListNode(0);
	//这一步非常关键,否则在加入头结点的操作中会出现null.next的错误!!!
	head.next=tail;
	tail.prev=head;
		}

	/** Get the value of the index-th node in the linked list. If the index is invalid, return -1. */
		public int get(int index) {
	if(index < 0 \|\| index >= size){return -1;}
	ListNode cur = head;

	// 通过判断 index < (size - 1) / 2 来决定是从头结点还是尾节点遍历,提高效率
	if(index < (size - 1) / 2){
	for(int i = 0; i <= index; i++){
	cur = cur.next;
	}
	}else{
	//判断index是否有效
	if(index<0 \|\| index>=size){
	return -1;
	}
	ListNode cur = this.head;
	//判断是哪一边遍历时间更短
	if(index >= size / 2){
	//tail开始
		cur = tail;
	for(int i = 0; i <= size - index - 1; i++){
	for(int i=0; i< size-index; i++){
		cur = cur.prev;
		}
	}else{
	for(int i=0; i<= index; i++){
	cur = cur.next;
	}
		}
		return cur.val;
		}

	/** 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) {
	ListNode cur = head;
	ListNode newNode = new ListNode(val);
	newNode.next = cur.next;
	cur.next.prev = newNode;
	cur.next = newNode;
	newNode.prev = cur;
	size++;
	//等价于在第0个元素前添加
	addAtIndex(0,val);
		}

	/** Append a node of value val to the last element of the linked list. */
		public void addAtTail(int val) {
	ListNode cur = tail;
	ListNode newNode = new ListNode(val);
	newNode.next = tail;
	newNode.prev = cur.prev;
	cur.prev.next = newNode;
	cur.prev = newNode;
	size++;
	//等价于在最后一个元素(null)前添加
	addAtIndex(size,val);
		}

	/** 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) {
	if(index > size){return;}
	if(index < 0){index = 0;}
	ListNode cur = head;
	for(int i = 0; i < index; i++){
	cur = cur.next;
	//index大于链表长度
	if(index>size){
	return;
	}
	//index小于0
	if(index<0){
	index = 0;
		}
	ListNode newNode = new ListNode(val);
	newNode.next = cur.next;
	cur.next.prev = newNode;
	newNode.prev = cur;
	cur.next = newNode;
		size++;
	//找到前驱
	ListNode pre = this.head;
	for(int i=0; i<index; i++){
	pre = pre.next;
	}
	//新建结点
	ListNode newNode = new ListNode(val);
	newNode.next = pre.next;
	pre.next.prev = newNode;
	newNode.prev = pre;
	pre.next = newNode;

		}

	/** Delete the index-th node in the linked list, if the index is valid. */
		public void deleteAtIndex(int index) {
	if(index >= size \|\| index < 0){return;}
	ListNode cur = head;
	for(int i = 0; i < index; i++){
	cur = cur.next;
	//判断索引是否有效
	if(index<0 \|\| index>=size){
	return;
		}
	cur.next.next.prev = cur;
	cur.next = cur.next.next;
	//删除操作
		size--;
	ListNode pre = this.head;
	for(int i=0; i<index; i++){
	pre = pre.next;
	}
	pre.next.next.prev = pre;
	pre.next = pre.next.next;
		}
		}

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