LeetCode707.设计链表

力扣

设计链表的五个接口：

• 获取链表第index个节点的数值

• 在链表的最前面插入一个节点

• 在链表的最后面插入一个节点

• 在链表第index个节点前面插入一个节点

• 删除链表的第index个节点

//定义结点
public 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 {

    int size;
    ListNode dummy;

    //初始化链表
    //定义了一个虚拟头结点
    public MyLinkedList() {
        size = 0;
        dummy = new ListNode(0);
    }
    
    //获得第index个结点的数值
    //注意index是从0开始的，第0个结点就是头结点
    public int get(int index) {
        
        if (index < 0 || index >= size) return -1;

        ListNode cur = dummy;

        for (int i = 0; i <= index; i ++) cur = cur.next;
        return cur.val;

    }
    
    // 在链表最前面插入一个结点
    public void addAtHead(int val) {
        ListNode newNode = new ListNode(val);

        //新结点指向虚拟结点的下一个
        newNode.next = dummy.next;

        //虚拟结点指向新结点
        dummy.next = newNode;
        size++;
    }
    
    public void addAtTail(int val) {

        ListNode newNode = new ListNode(val);
        ListNode cur = dummy;

        while (cur.next != null) {
            cur = cur.next;
        }
        cur.next = newNode;
        size++;

    }
    
    public void addAtIndex(int index, int val) {

        //如果index大于链表的长度，则返回空
        if (index > size) return;

        //如果index小于0，则置为0，作为链表的新头结点
        if (index < 0) index = 0;

        //插入结点
        ListNode newNode = new ListNode(val);
        ListNode cur = dummy;

        while (index != 0) {
            cur = cur.next;
            index --;
        }

        newNode.next = cur.next;
        cur.next = newNode;
        size++;
    }
    
    public void deleteAtIndex(int index) {
        if (index >= size || index < 0) return;
        
        size --;

        ListNode cur = dummy;

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

        }
        */

        while (index != 0) {
            cur = cur.next;
            index --;
        }

        cur.next = cur.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);
 */