力扣——707. 设计链表

class MyLinkedList {
    int size;
    ListNode head, tail;
    public MyLinkedList() {
        size = 0;
        head=tail=new ListNode(0);
        head.next=null;
        tail.prev=null;
    }

    public int get(int index) {
        if(index<0||index>=size){
            return -1;
        }
        int count=0;
        ListNode temp;
        if (index<size/2){
            temp=head;
            while(count<=index){
                if(temp.next==null){
                    break;
                }
                temp=temp.next;
                count++;
            }
        }else {
            temp=tail;
            while (count<(size-index-1)){
                temp=temp.prev;
                count++;
            }
        }
        return temp.val;
    }

    public void addAtHead(int val) {
        ListNode temp = new ListNode(val);
        if(head.next==null){
            head.next=temp;
            temp.prev=head;
            tail=temp;
        }else{
            head.next.prev=temp;
            temp.next=head.next;
            head.next=temp;
            temp.prev=head;
        }
        size++;
    }

    public void addAtTail(int val) {
        ListNode temp = new ListNode(val);
        tail.next=temp;
        temp.prev=tail;
        tail=tail.next;
        size++;
    }

    public void addAtIndex(int index, int val) {
        if(index<0){
            addAtHead(val);
        }else if(index==size){
            addAtTail(val);
        }else if(index>=size){
            return;
        }else{
            int count=0;
            ListNode temp1 = new ListNode(val);
            ListNode temp;
            if(index<size/2){
                temp=head;
                while(count<=index){
                    temp=temp.next;
                    count++;
                }
            }else{
                temp=tail;
                while (count<(size-index-1)){
                    temp=temp.prev;
                    count++;
                }   
            }
            temp1.next=temp;
            temp1.prev=temp.prev;
            temp.prev.next=temp1;
            temp.prev=temp1;
            size++;
        }
    }

    public void deleteAtIndex(int index) {
        if(index<0||index>=size){
            return;
        }
        int count=0;
        ListNode temp;
        if(index<size/2){
            temp=head;
            while(count<=index){
                temp=temp.next;
                count++;
            }
        }else{
            temp=tail;
            while (count<(size-index-1)){
                temp=temp.prev;
                count++;
            } 
        }
        if(temp.next==null){
            tail=tail.prev;
            temp.prev.next=null;
            temp.prev=null;
        }else {
            temp.prev.next=temp.next;
            temp.next.prev=temp.prev;
        }
        size--;
    }
}
public class ListNode {
  int val;
  ListNode next;
  ListNode prev;
  ListNode(int x) { val = x; }
}

/**
 * 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);
 */