707. 设计链表
     

class ListNode {
    int val;
    ListNode next;

    public ListNode() {

    }

    public ListNode(int val) {
        this.val = val;
    }

    public ListNode(int val, ListNode next) {
        this.val = val;
        this.next = next;
    }

}

class MyLinkedList {

    //size存储链表元素的个数
    int size;
    //虚拟头节点
    ListNode head;

    //初始化链表
    public MyLinkedList() {
        size = 0;
        head = new ListNode(0);
    }
    
    //获取第index个节点的数值，注意index是从0开始的，第0个节点就是头结点
    public int get(int index) {
        if (index < 0 || index >= size) {
            return -1;
        }
        //当前指针先指向虚拟头节点
        ListNode cur = head;

        for (int i = 0; i <= index; i++) {
            cur = cur.next;
        }

        return cur.val;
    }
    
     //在链表最前面插入一个节点，等价于在第0个元素前添加
    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;
        }

        if (index < 0) {
            index = 0;
        }

        size++;

        ListNode pre = head;
        
        for (int i = 0; i < index; i++) {//举个极端的在末尾插入的例子，如数组[1]，在index为1处插入元素，循环结束会出现pre指向null的情况，导致引用失效，所以i不能=index
            pre = pre.next;
        }

        ListNode toAdd = new ListNode(val);
        toAdd.next = pre.next;
        pre.next = toAdd;
    }
    
    public void deleteAtIndex(int index) {
        if (index < 0 || index >= size) {
            return;
        }

        size--;

        if (index == 0) {
            head = head.next;
            return;
        }

        ListNode pre = head;
        for (int i = 0; i < index; i++) {//极端例子，考虑到pre和pre.next都不能为空；如[1,2,3]删除index为2的元素，
            pre = pre.next;
        }

        pre.next = pre.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);
 */