如何检查链表中是否存在元素？

Question

I have to check if an element is present in a linkedlist or not? I wrote the following code but it is not returning the right output.I m unable to find what I did wrong.Can anyone tell what I did wrong in the following code? The expected output is false and true but the output I am getting is false and false.

package BasicList1;


import java.util.Vector;

public class BasicList1 implements ListInterface{

    static String[] testcase1 = {"3","1","2","6","7","4","5"};

    public static void main (String[] args){
        BasicList1 testInstance = new BasicList1();
        ListNode head = new ListNode(testcase1[0]);
        ListNode node = head;
        System.out.println(testInstance.elements(head));
        System.out.println(testInstance.hasElement(head, "9"));
        System.out.println(testInstance.hasElement(head,"4"));
    }

    public BasicList1 getBasicList(String data){
        return this;
    }

    //write your code here
    public Vector<String> elements(ListNode head){
        ListNode temp=head;
        Vector v=new Vector();
        while(temp!=null){
            v.addElement(temp);
            temp=temp.next;
        }
        return v;
    }

    @Override
    public boolean hasElement(ListNode head, String data) {
        ListNode temp=head;
        while(temp!=null){
            if(temp.data.equals(data)){
                return true;
            }
            temp=temp.next;
        }
        return false;
    }

Answer 1

I do not know what your wrong output is, but I guess your ListNode only contains a single element: 3.

This only creates a ListNode with a single node containing testcase1[0] (which is "3" ):

ListNode head = new ListNode(testcase1[0]);

So, to initialize your ListNode , you should write something like that:

ListNode head = new ListNode(testcase1[testcase1.length - 1]); 
for (int i = testcase1.length - 2; i >= 0; i--) {
     final ListNode tail = head;
     head = new ListNode(testcase1[i]);
     head.next = tail;
}

The NodeList is built in the reversed order to avoid walking through the whole list to add elements at the end.

EDIT

There is also a problem within your elements method. You're adding the ListNode instead of its contents, so replace

v.addElement(temp);

with

v.addElement(temp.data);

EDIT 2

Moreover, the BasicList1 design is quite bad: it is not really object oriented. You should have something like this:

package test;

import java.util.Vector;

public class BasicList1 {

    private ListNode head;

    public BasicList1(String[] data) {

        if (data.length != 0 ) {
            head = new ListNode(data[data.length - 1]);
        }

        for (int i = data.length - 2; i >= 0; i--) {
            final ListNode tail = head;
            head = new ListNode(data[i]);
            head.next = tail;
        }
    }

    public BasicList1 getBasicList() {
        return this;
    }

    //write your code here
    public Vector<String> elements(){
        ListNode temp = head;
        Vector<String> v=new Vector<String>();
        while(temp!=null){
            v.addElement(temp.data);
            temp=temp.next;
        }
        return v;
    }

    @Override
    public boolean hasElement(String data) {
        ListNode temp=head;
        while(temp!=null){
            if(temp.data.equals(data)){
                return true;
            }
            temp=temp.next;
        }
        return false;
    }

    public static void main (String[] args){
        final String[] testcase1 = {"3","1","2","6","7","4","5"};
        final BasicList1 testInstance = new BasicList1(testcase1);

        System.out.println(testInstance.elements());
        System.out.println(testInstance.hasElement("9"));
        System.out.println(testInstance.hasElement("4"));
    }
}