一种递归算法，用于添加非哑头单向线性列表的所有元素。 只有列表的头部将作为参数给出

Question

I am facing a run-time error while adding the elements of a singly linked list. I have po enter image description here sted the picture of the error.I am attaching my codes here:

    public class Node{
      Node next;
      int a=0;
    
        public Node (int e, Node n){
          a =e;
          next =n;
      }}
    
    
    public class Question6{
      static int sum=0;
      public static void main(String[]args){
    
        Node head=null;
        
        Node n5= new Node(5,null);
        Node n4= new Node(4,n5);
        Node n3= new Node(3,n4);
        Node n2= new Node(2,n3);
        Node n1= new Node(1,n2);
        
        head=n1;
        sum =add(head);   
      }
      public static int add(Node head){
        if(head==null){
          return sum;
        }
        else{
          sum=sum + head.a;
          head=head.next;
          add(head);
        }
        return sum;
      }
    }

Answer 1

It looks like something wrong with your compiler. Error in the image says that it can not find init method in class Node . Try to add empty init method (curly brackets) in class Node :

class Node{
        Node next;
        int a = 0;

        {}

        public Node(int e, Node n) {
            a = e;
            next = n;
        }
}

Answer 2

First of all your code is not a real recursion, because of you use external static variable .

public static int sum(Node head) {
    return sum(head, 0);
}

private static int sum(Node node, int sum) {
    return node == null ? sum : sum(node.next, sum + node.a);
}