如何在Java中从双向链接列表中随机删除任何节点？

Question

I'm making a Doubly Linked List that allows you to insert at the front and rear, as well as deleting any node from the list as long as it exists. The problem is that it doesn't work and gives off and either gives off a NullPointerException or it just says That Integer does not exist even though it does exist.The code is:

public class Numbers {

    Node head = null; //Head of the list
    Node tail = null; //end of the doubly list    
    int size = 0;


    public void FrontInsert(int data) {
        Node n = new Node();
        if (head == null) {
            head = n;

        } else {
            n.prev = head;
            head.next = n;
            head = n;

        }
        size++;
    }

    public void RearInsert(int data) {
        Node n = new Node();
        if (head == null) {
            head = n;
            tail = n;

        } else {

            n.next = tail;
            tail.prev = n;
            tail = n;
        }
        size++;
    }

    public void Delete(int x) {

        if (size == 0) {
            System.out.println("The list is empty.");
        }
        if (head.data == x) {
            head = head.next;
            if (head != null) {
                head.prev = null;
            }
            size--;
            return;
        }

        tmp = head;

        while (tmp != null && tmp.data != x) {
            tmp = tmp.next;
        }

        if (tmp == null) {
            System.out.println("That integer does not exist.");
            return;
        }

        if (tmp.data == x) {
            tmp.prev.next = tmp.next;
            if (tmp.next != null) {
                tmp.next.prev = tmp.prev;
            }
        }
        size--;
    }
    public void printList() {
        while (head != null) {
            System.out.print(head.data + " ");
            head = head.prev;
        }
    }
    public static void main(String[] args) {
        Numbers nu = new Numbers();

    }
    class Node {
        Node prev;
        Node next;
        int data;

        public void Node(int data) {
            this.data = data;
            next = null;
            prev = null;
        }
    }
}    

Answer 1

Try this and check output

public class Numbers {

Node head = null; 
Node tail = null; 
int size = 0;

public void FrontInsert(int data) {
    Node n = new Node(data);
    if (head == null) { // first insert
        head = n;
        tail = n;
    } else {
        n.next = head;
        head.prev = n;
        head = n;

    }
    size++;
}

public void RearInsert(int data) {
    Node n = new Node(data);
    if (head == null) {
        head = n;
        tail = n;

    } else {

        n.prev = tail;
        tail.next = n;
        tail = n;
    }
    size++;
}

public void Delete(int index) { // index is the position to be remove

    if (size == 0) {
        System.out.println("The list is empty."); return;
    }else if(index < 0 || index > size -1){
        System.out.println("Index outOf Bound."); return;
    }

    Node currentNode  = head;
    for(int i = 1; i <= index ; i++){
        currentNode = currentNode.next;
    }
    //remove
    if (index == 0) {
        currentNode.next.prev = null;
        head = currentNode.next;
    } else if (index == size - 1) {
        currentNode.prev.next = null;
        tail = currentNode.prev;
    } else {
        if (currentNode.prev != null) // Ensure its not header
            currentNode.prev.next = currentNode.next;
        if (currentNode.next != null) // Ensure its not tail
            currentNode.next.prev = currentNode.prev;
    }
    size--;
}

public void printList() {
    Node tmp = head;
    while (tmp != null) {
        System.out.print(tmp.data + "  ");
        tmp = tmp.next;
    }
    System.out.println();
}

public static void main(String[] args) {
    Numbers nu = new Numbers();
    nu.FrontInsert(1);nu.printList();
    nu.FrontInsert(2);nu.printList();
    nu.RearInsert(3);nu.printList();
    nu.FrontInsert(4);nu.printList();
    nu.RearInsert(3);nu.printList();
    nu.FrontInsert(4);nu.printList();
    nu.RearInsert(3);nu.printList();
    nu.RearInsert(3);nu.printList();
    nu.FrontInsert(4);nu.printList();
    System.out.println();
    nu.Delete(4);
    nu.printList();
}

class Node {
    Node prev;
    Node next;
    int data;

    public Node(int data) {
        this.data = data;
        next = null;
        prev = null;
    }
}

}

Answer 2

Well, your head and tail were mutually exclusive, i mean when you add something to the tail of the list, you were only giving one side reference not both side, you have to says

tail.next = n; n.prev = tail; and tail = n tail.next = n; n.prev = tail; and tail = n .

Here is a working code:

public class Numbers {

Node head = null; //Head of the list
Node tail = null; //end of the doubly list    
int size = 0;


public void FrontInsert(int data) {
    Node n = new Node(data);
    if (head == null) {
        head = n;
        tail = head;
    } else {
        n.next = head;
        head.prev = n;
        head = n;

    }
    size++;
}

public void RearInsert(int data) {
    Node n = new Node(data);
    if (head == null) {
        head = n;
        tail = head;
    } else {
        n.next = null;
        tail.next = n;
        n.prev = tail;
        tail = n;
    }
    size++;
}

@SuppressWarnings("null")
public void Delete(int x) {

    if (size == 0) {
        System.out.println("The list is empty.");
        return;
    }
    if (head.data == x) {
        head = head.next;
        if (head != null) {
            head.prev = null;
        }
        size--;
        return;
    }

    Node tmp = head;


    while (true) {
        if(tmp == null)
            break;
        if(tmp.data == x)
            break;
        System.out.println(tmp.data);
        tmp = tmp.next;
    }

    if (tmp == null) {
        System.out.println("That integer does not exist.");
        return;
    }

    if (tmp.data == x) {
        tmp.prev.next = tmp.next;
        if (tmp.next != null) {
            tmp.next.prev = tmp.prev;
        }
    }
    size--;
}
public void printList() {
    while (head != null) {
        System.out.print(head.data + " ");
        head = head.next;
    }
}
public static void main(String[] args) {
    Numbers nu = new Numbers();
    nu.FrontInsert(2);
    nu.FrontInsert(3);   
    nu.FrontInsert(6);
    nu.RearInsert(8);
    nu.RearInsert(20);
    nu.Delete(8);
    nu.printList(); 
   // System.out.println(nu.head.data + "data");
 //   System.out.println(nu.head.next.data + "data");

}
class Node {
    Node prev;
    Node next;
    private int data;


    public Node(int data) {
        this.data = data;
        next = null;
        prev = null;
    }
}

}