自己写一个简单的LinkedList

单链表

推荐阅读：https://www.cnblogs.com/zwtblog/tag/源码/

• 哨兵节点：

哨兵节点在树和链表中被广泛用作伪头、伪尾等，通常不保存任何数据。

我们将使用伪头来简化我们简化插入和删除。

class ListNode {
    int val;
    ListNode next;

    ListNode() {
    }

    /**
     * @param val
     */
    ListNode(int val) {
        this.val = val;
    }

    /**
     * @param val
     * @param next
     */
    ListNode(int val, ListNode next) {
        this.val = val;
        this.next = next;
    }
}

/**
 * MyLinkedList
 */
class MyLinkedList {
    int size;
    ListNode head;  //虚节点

    public MyLinkedList() {
        size = 0;
        head = new ListNode(0);
    }

    /**
     * @param index
     * @return
     */
    public int get(int index) {
        if (index < 0 || index >= size) {
            return -1;
        }
        ListNode cur = head;
        for (int i = 0; i < index + 1; i++) {
            cur = cur.next;
        }
        return cur.val;
    }

    /**
     * @param val
     */
    public void addAtHead(int val) {
        addAtIndex(0, val);
    }

    /**
     * @param val
     */
    public void addAtTail(int val) {
        addAtIndex(size, val);
    }

    /**
     * @param index
     * @param 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++) {
            pre = pre.next;
        }
        ListNode add = new ListNode(val);
        add.next = pre.next;
        pre.next = add;
    }

    /**
     * @param index
     */
    public void deleteAtIndex(int index) {
        if (index < 0 || index >= size) return;
        size--;
        ListNode pre = head;
        for (int i = 0; i < index; i++) {
            pre = pre.next;
        }
        pre.next = pre.next.next;
    }
}

public class LC707 {
    public static void main(String[] args) {
        MyLinkedList linkedList = new MyLinkedList();
        linkedList.addAtHead(1);
        linkedList.addAtTail(3);
        linkedList.addAtIndex(1, 2);   //链表变为1-> 2-> 3
        System.out.println(linkedList.get(1));            //返回2
        linkedList.deleteAtIndex(1);  //现在链表是1-> 3
        System.out.println(linkedList.get(1));            //返回3
    }
}