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I am practicing my algorithms and i am attempting a problem to delete even values in a linked list but it fails. When attempting to print the linked list class after deleting the even values as i provided test cases to see if the program works correctly. It prints out 10, and 1. 11 and 1 should be the values printed instead. 

public class LinkedList
{

  private Node head;

  public LinkedList() {
   this.head = null; 
  }

  public void add(int data) {
    Node node = new Node(data);
    if(head == null) {
        node.setNextNode(head);
        head = node;
    }
    else {
       Node curr = head;
      while(curr.next != null) {
       curr = curr.getNextNode(); 
      }
      curr.setNextNode(node);
    }
  }

  public void removeEven() {
   if (this.head==null) return;

    Node prev=null, curr = head;

    while(curr != null) {
      if(curr.data%2 == 0) {
        if(prev == null) {
           Node node = curr.getNextNode();
           node.setNextNode(prev);
           prev = node;
           curr = curr.getNextNode().getNextNode();
        }
        else {
           Node node = curr.getNextNode();
           prev.setNextNode(node);
           curr = curr.getNextNode();
        }
       // curr = curr.getNextNode();
      }

      else{
        prev = curr;
        curr = curr.getNextNode();
      }
    }
  }

  public void print() {
   if(head == null) return;

    Node curr = head;
    while(curr != null) {
      System.out.println(curr);
      curr = curr.getNextNode(); 
    }
  }

  public static void main(String[] args)
  {
     LinkedList h = new LinkedList();
     h.add(10);
     h.add(1);
     h.add(11);
     h.add(8);
     h.removeEven();
     h.print();
  }
}

class Node {
  int data;
  Node next;

  public Node(int data) {
    this(data, null);
  } 

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

  public void setNextNode(Node next) {
    this.next = next; 
  }

  public Node getNextNode() {
    return this.next; 
  }

  @Override
  public String toString() {
    return "Data: "+this.data; 
  }
}
EI-01
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    The problem is the code after `if(prev == null) {`. To remove the first element of the list you should set `head` to `curr.getNextNode()`. Instead you do `curr.getNextNode().setNextNode(null)`. – Tesseract Apr 17 '17 at 06:10
  • You should read up on debugging Java programs. Look at [one](http://stackoverflow.com/questions/426569/why-is-debugging-better-in-an-ide) and [two](http://stackoverflow.com/questions/17630527/how-to-debug-a-java-program-without-using-an-ide). And in your program, give each method one responsibility: make one method to remove nodes, and another method to select the nodes with even numbers, and to remove them using the first method. Right now those two logics are in one method making it hard to debug. Hint: the problem is most likely in your removal code. – Erwin Bolwidt Apr 17 '17 at 06:10
  • @ErwinBolwidt yes the problem is in my removal method – EI-01 Apr 17 '17 at 06:29

1 Answers1

1
public class LinkedList
{

  private Node head;

  public LinkedList() {
   this.head = null; 
  }

  public void add(int data) {
    Node node = new Node(data);
    if(head == null) {
        node.setNextNode(head);
        head = node;
    }
    else {
       Node curr = head;
      while(curr.next != null) {
       curr = curr.getNextNode(); 
      }
      curr.setNextNode(node);
    }
  }

  public void removeEven() {
   if (this.head==null) return;

    Node prev=null, curr = head;

    while(curr != null) {
      if(curr.data%2 == 0) {
        if(prev == null) {
           Node node = curr.getNextNode();
           curr.setNextNode(prev);
           prev = node;
           curr = prev.getNextNode();
           head = prev;
        }
        else {
           Node node = curr.getNextNode();
           prev.setNextNode(node);
           curr.setNextNode(null);
           curr = node;
        }
      }

      else{
        prev = curr;
        curr = curr.getNextNode();
      }
    }
  }

  public void print() {
   if(head == null) return;

    Node curr = head;
    while(curr != null) {
      System.out.println(curr);
      curr = curr.getNextNode(); 
    }
  }

  public static void main(String[] args)
  {
     LinkedList h = new LinkedList();
     h.add(10);
     h.add(1);
     h.add(11);
     h.add(8);
     h.removeEven();
     h.print();
  }
}

class Node {
  int data;
  Node next;

  public Node(int data) {
    this(data, null);
  } 

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

  public void setNextNode(Node next) {
    this.next = next; 
  }

  public Node getNextNode() {
    return this.next; 
  }

  @Override
  public String toString() {
    return "Data: "+this.data; 
  }
}
Ishank Gulati
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