No enclosing instance of type ReverseList is accessible. Must qualify the allocation with an enclosing instance of type ReverseList (e.g. x.new A() where x is an instance of ReverseList).

package day2;
//自己定义双向链表 和 单链表结构 然后进行反转操作

public class ReverseList {


    public  class Node {
        public int data ;
        public Node next;
        
        public Node (int data) {
            this.data = data;
            //this.next = null;
        }    
    }
    
    
    //反转单项链表
    public static Node reverseList(Node head ) {
        // 三个指针的
        Node next = null;
        Node pre = null;
        while (head != null) {
            next = head.next;
            head.next = pre;
            pre = head;
            head = next;
        }
        return pre;
    }
    
    
    
    
    
    public  class DoubleNode{
        public int data ;
        public DoubleNode next;
        public DoubleNode pre;
        
        public DoubleNode (int data){
            this.data = data;
            this.next = null;
            this.pre = null;
        }
    }
    

    //反转双向链表
    public static DoubleNode reverseDoubleList(DoubleNode head) {
        DoubleNode pre = null;
        DoubleNode next = null;
        while (head != null) {
            next = head.next;
            head.next = pre;
            head.pre = next;
            pre = head ;
            head = next;
        }
        return pre;
    }

    
    public static void printLinkedList(Node head) {
        System.out.print("Linked List: ");
        while (head != null) {
            System.out.print(head.data+" -> ");
            head=head.next;
        }
        System.out.println("null");
    }
    
    public static void printDoubleLinkedList(DoubleNode head) {
        System.out.print("Double Linked List: ");
        DoubleNode end = null;
        while (head != null) {
            System.out.print(head.data+" -> ");
            end = head;
            head=head.next;
        }
        System.out.print("null | ");
        
        
        while (end != null) {
            System.out.print(end.data+" -> ");
            //end = head;
            end = end.pre;
        }
        System.out.println("null");
    }
    
    
    public static void main(String[] args) {
        Node head1 = new Node(1);
        //Node head1 = new Node(1);
        head1.next = new Node(2);
        head1.next.next = new Node(3);
        printLinkedList(head1);
        head1 = reverseList(head1);
        printLinkedList(head1);

        DoubleNode head2 = new DoubleNode(1);
        head2.next = new DoubleNode(2);
        head2.next.pre = head2;
        head2.next.next = new DoubleNode(3);
        head2.next.next.pre = head2.next;
        head2.next.next.next = new DoubleNode(4);
        head2.next.next.next.pre = head2.next.next;
        printDoubleLinkedList(head2);
        
        printDoubleLinkedList(reverseDoubleList(head2));

    }


    
    

}

我写的内部类是动态的，也就是开头以public class开头。而主程序是public static class main。在Java中，类中的静态方法不能直接调用动态方法。只有将某个内部类修饰为静态类，然后才能够在静态类中调用该类的成员变量与成员方法。所以在不做其他变动的情况下，最简单的解决办法是将public class改为public static class.

修改：

package day2;
//自己定义双向链表 和 单链表结构 然后进行反转操作

public class ReverseList {


    public static class Node {
        public int data ;
        public Node next;
        
        public Node (int data) {
            this.data = data;
            //this.next = null;
        }    
    }
    
    
    
    
    public static class DoubleNode{
        public int data ;
        public DoubleNode next;
        public DoubleNode pre;
        
        public DoubleNode (int data){
            this.data = data;
            this.next = null;
            this.pre = null;
        }
    }
    


    
    

}