class LN{
    int data;
    public LN next;
    public LN(int data) {
        this.data = data;
        this.next = null;
    }  
}

public class ListPractice {

    static LN head = null;
    public static int length;
    
    public static void main(String[] args) {
       
        int[] nums = {1,2,3,4};
        for (int i = 0; i < nums.length; i++) {
            insertLN(nums[i]);
        }
        deleteLN(1);
        deleteLNIndex(2);
        LN q = head;
        while(q!=null){
            System.out.println(q.data);
            q = q.next;
        }
    }
    
    //插入
    public static void insertLN(int num){
        
        LN p = new LN(num);
        
        if(head == null){
            head = p;
            length++;
            return;
        }
        
        LN q = head;
        while(q.next!=null){
            q = q.next;
        }
        q.next = p;
        length++;
    }
    
    //按key值删除
    public static void deleteLN(int num){
        if(head != null){
            
            LN p = head;
            LN q = p.next;
            
            if(p.data == num){
                head = q;
            }
            
            while(q!=null){
                if(q.data==num){
                    p.next = q.next;
                    return;
                }
                p = q;
                q = q.next;
            }
        }
    }
    
    
    //按索引删除
    public static void deleteLNIndex(int index){
        
        LN p = head;
        LN q = p.next;
        
        if(index<1||index>length){
            return;
        }
        
        if(index == 1){
            head = head.next;
            return;
        }
        
        for (int i = 2; i < index; i++) {
            p = q;
            q = q.next;
        }
        p.next = q.next;
    }
}