707. Design Linked List

Design your implementation of the linked list. You can choose to use a singly or doubly linked list.
A node in a singly linked list should have two attributes: val and next. val is the value of the current node, and next is a pointer/reference to the next node.
If you want to use the doubly linked list, you will need one more attribute prev to indicate the previous node in the linked list. Assume all nodes in the linked list are 0-indexed.

Implement the MyLinkedList class:

• MyLinkedList() Initializes the MyLinkedList object.
• int get(int index) Get the value of the indexth node in the linked list. If the index is invalid, return -1.
• void addAtHead(int 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.
• void addAtTail(int val) Append a node of value val as the last element of the linked list.
• void addAtIndex(int index, int val) Add a node of value val before the indexth node in the linked list. If index equals the length of the linked list, the node will be appended to the end of the linked list. If index is greater than the length, the node will not be inserted.
• void deleteAtIndex(int index) Delete the indexth node in the linked list, if the index is valid.

单链表，设置一个哨兵节点在头部。
设置辅助方法，取得index的前一个节点。
addAtHead和addAtTail都可以采用addAtIndex实现。

采用双链表速度可以更快。

class MyLinkedList {
    ListNode head;
    int size;
    public MyLinkedList() {
        ListNode dummy = new ListNode(0);
        head = dummy;
        size = 0;
    }
    
    public ListNode getPrev(int index){
        ListNode prev = head;
        for(int i = 0; i < index; i++){
            prev = prev.next;
        }
        return prev;
    }
    
    public int get(int index) {
        if(index >= size) return -1;
        return getPrev(index).next.val;
    }
    
    public void addAtHead(int val) {
        addAtIndex(0, val);
    }
    
    public void addAtTail(int val) {
        addAtIndex(size, val);
    }
    
    public void addAtIndex(int index, int val) {
        if(index > size) return;
        ListNode prev = getPrev(index);
        ListNode temp = prev.next;
        prev.next = new ListNode(val);
        prev.next.next = temp;
        size++;
    }
    
    public void deleteAtIndex(int index) {
        if(index >= size) return;
        if(index == size-1){
            ListNode prev = getPrev(index);
            prev.next = null;
            size--;
            return;
        }
        ListNode prev = getPrev(index);
        prev.next = prev.next.next;
        size--;
    }
    
    private void print(){
        ListNode root = head.next;
        while(root != null){
            System.out.print(root.val+" ->   ");
            root = root.next;
        }
        System.out.println("Size:   "+size);
    }
    
    class ListNode{     
        int val;
        ListNode next;
        public ListNode(int v){
            val = v;
        }
    }
}

/**
 * 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);
 */