707、设计链表

class ListNode{//单向链表
    int val;
    ListNode next;
    ListNode(){}
    ListNode(int val){
        this.val = val;
    }
}
class MyLinkedList {
    int size;//存储链表元素的个数
    ListNode head;//虚拟头结点
    public MyLinkedList() {//初始化链表
        size = 0;
        head = new ListNode(0);
    }
    
    public int get(int index) {//获取第index个节点的数值
        if (index < 0 || index >= size){
            return -1;
        }
        ListNode currentNode = head;
        for (int i = 0; i <= index; i++){
            currentNode = currentNode.next;
        }
        return currentNode.val;
    }
    
    public void addAtHead(int val) {//在链表最前面插入一个节点
        addAtIndex(0, val);
    }
    
    public void addAtTail(int val) {//在链表的最后插入节点
        addAtIndex(size, val);
    }
    
    public void addAtIndex(int index, int val) {
        if (index > size){
            return;
        }
        if (index < 0){
            index = 0;
        }
        size++;//插入一个节点链表变长

        ListNode pred = head;
        for (int i = 0; i < index; i++){
            PRed = pred.next;//找到要插入节点的前驱
        }
        ListNode toAdd = new ListNode(val);
        toAdd.next = pred.next;
        pred.next = toAdd;
    }
    
    public void deleteAtIndex(int index) {
        if (index < 0 || index >= size){
            return;
        }
        size--;
        ListNode pred = head;
        for (int i = 0; i < index; i++){
            pred = pred.next;
        }
        pred.next = pred.next.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);
 */

class ListNode{//双向链表
    int val;
    ListNode next, prev;
    ListNode(int x){val = x;}
    
}
class MyLinkedList {
    int size;//存储链表元素的个数
    ListNode head;//虚拟头结点
    ListNode tail;
    public MyLinkedList() {//初始化链表
        size = 0;
        head = new ListNode(0);
        tail = new ListNode(0);
        head.next = tail;
        tail.prev = head;
    }
    
    public int get(int index) {//获取第index个节点的数值
        if (index < 0 || index >= size){
            return -1;
        }
        ListNode currentNode = head;
        if (index < (size -1)/2){  // 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
            for (int i = 0; i <= index; i++){
                currentNode = currentNode.next;
            }
        }else {
            currentNode = tail;
            for (int i = 0; i <= size - index - 1; i++){
                currentNode = currentNode.prev;
            }
        }
        
        return currentNode.val;
    }
    
    public void addAtHead(int val) {//在链表最前面插入一个节点
        addAtIndex(0, val);
    }
    
    public void addAtTail(int val) {//在链表的最后插入节点
        addAtIndex(size, val);
    }
    
    public void addAtIndex(int index, int val) {
        if (index > size){
            return;
        }
        if (index < 0){
            index = 0;
        }
        size++;//插入一个节点链表变长

        ListNode pred = head;
        for (int i = 0; i < index; i++){
            pred = pred.next;//找到要插入节点的前驱
        }
        ListNode toAdd = new ListNode(val);
        toAdd.next = pred.next;
        pred.next.prev = toAdd;
        toAdd.prev = pred;
        pred.next = toAdd;
    }
    
    public void deleteAtIndex(int index) {
        if (index < 0 || index >= size){
            return;
        }
        size--;
        ListNode pred = head;
        for (int i = 0; i < index; i++){
            pred = pred.next;
        }
        pred.next.next.prev = pred;
        pred.next = pred.next.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);
 */