title: How to find Intersections in two singly linked lists
body: <p>I am writing an program in which i have to set up data structure dictionary(singly linked list) with words alphabetically ordered(words that appear in sentence in a text document with document ids). and find which words appear in more than one document so the professor wants us to do an intersection.  I am really confused on how to do the intersection.  I have everything else(Which I believe is correct).  Here is my code(I have added my intersect algorithm, but it is clearly not working and I followed the professors algorithm[she never shows us an example]):</p>
  
  <pre><code>public class dictionary 
  {
    //variables
    dNode head;
    int size;
  
    //constructor
    public dictionary() 
    {
      head = null;
      size = 0;
    }
  
    //addFirst method
    public void addFirst(dNode s) 
    {
      s.setNext(head);
      head = s;
      size++;
    }
  
    public void addLast(dNode s)
    {
      if ( head == null )
      {
        head = s;
      }
      else
      {
        s.setNext(null);
        dNode w = head;
        while ( w.getNext() != null ) 
        { 
          w = w.getNext();
        }
        w.setNext(s);
      }
        size++;
    }
  
    //toString Method
    public String toString() 
    {
      String w = "";
      dNode s = head;
      while ( s != null ) 
      {
        w += s + "\n";
        s = s.getNext();
      }
      return w;
    }
  
    //intersection method
  public String intersection(pNode head, dNode head) {
  int left = posting.head;
  int right = dictionary.head;
  int result = new dictionary();
  
  while (left != null &amp;&amp; right != null) {
       if (dID.left &lt; dID.right) {
       left = left.next;
  else if (dID.left &gt; dID.right)
       right = right.next;
  else 
       left = left.next;
       right = right.next;
       result.push(left.data() );
       }
  }
  return result;
  }  
  }
  
  
  public class dNode 
  {
    //variables
    String sent;
    posting post;
    dNode nextNode;
  
    //constructor
    public dNode(String sent, posting post, dNode nextNode)
    {
      this.sent = sent;
      this.post = post;
      this.nextNode = nextNode;
    }
  
    //returns element of this node
    public String getSent() {
      return sent;
    }
  
    //retunrs the next node of this node
    public dNode getNext() {
      return nextNode;
    }
  
    //modifier methods
    //sets elements of this node.
    public void setSent(String newSent) {
      sent = newSent;
    }
  
    //sets the next node of this node
    public void setNext( dNode newNext) {
      nextNode = newNext;
    }
    //toString method
    public String toString() 
    {
      return "Sentence and Posting: \n" + sent + "\n" + post;
    }
  }
  
  
  public class pNode {
    //variables
    int dID;
    String word;
    int occurence;
    pNode next;
  
    //constructor
    public pNode(int dID, String word, int occurence, pNode next)
    {
      this.dID = dID;
      this.word = word;
      this.occurence = occurence;
      this.next = next;
    }
    //return element of this node
    public String getWord() {
      return word;
    }
  
    //Returns the next node of this node
    public pNode getNext() {
      return next;
    }
  
    //Modifier methods
    //set the words of this node
    public void setWord(String newWord) {
      word = newWord;
    }
  
    //sets the next node of this node
    public void setNext(pNode newNext){
      next = newNext;
    }
  
    //toString method
    public String toString() {
      return "Document ID, Word, Occurence: \n " + dID + ", " 
        + word + ", " + occurence;
    }
  
  }
  
  
  public class posting 
  {
    //variables
    pNode head;
    int size;
  
    //constructor
    public posting() 
    {
      head = null;
      size = 0;
    }
  
    //addFirst method 
    public void addFirst(pNode s) 
    {
      s.setNext(head);
      head = s;
      size++;
    }
  
    //addLast method
    public void addLast(pNode s)
    {
      if ( head == null )
      {
        head = s;
      }
      else
      {
        s.setNext(null);
        pNode w = head;
        while ( w.getNext() != null ) 
        {
          w = w.getNext();
        }
        w.setNext(s);
      }
      size++;
    }
  
    //toString method
    public String toString()
    {
      String w = "";
      pNode s = head;
      while ( s != null) 
      {
        w += s + "\n";
        s = s.getNext();
      }
      return w;
    }
  }
  
  import java.io.*;
  import java.util.*;
  
    public class testFile
    {
  
    public static void main (String[] args) throws FileNotFoundException 
    {
      File filename = new File("/export/home/hawkdom2/s0878044/CS503/assignment2/sentences.txt");
      Scanner scan = new Scanner(filename);
      dictionary Dictionary = new dictionary();
  
     while ( scan.hasNextLine() )
     {
       String sentence = scan.nextLine();
       String[] word = sentence.split(" ");
  
       //first element is document id
       int dID = Integer.parseInt( word[0] );
  
       //insertion sort
       for ( int i = 2; i &lt; word.length; i++ )
       {
         for ( int j = i; j &gt; 1; j-- )
         {
          if ( word[j].compareTo( word[j-1] ) &gt; 0 )
          {
            String switchs = word[j];
            word[j] = word[j-1];
            word[j-1] = switchs;
          }
         }
       }
  
       //integer array count
       int[] count = new int[word.length];
       for ( int i = 1; i &lt; word.length; i++)
       {
         for ( int j = 1; j &lt; word.length; j++)
         {
           if (word[i].equalsIgnoreCase( word[j] ) )
           {
             count[i]++;
           }
         }
       }
  
       posting posts = new posting();
  
       for ( int i = 1; i &lt; word.length; i++ )
       {
         if ( (i &gt; 1 ) &amp;&amp; (word[i].equalsIgnoreCase( word[i-1] ) ) )
           continue;
         else
         {
           posts.addFirst(new pNode(dID, word[i], count[i], null) );
         }
       }
  
       Dictionary.addLast(new dNode(sentence, posts, null) );
     }
  
  
  
  
  
     //print out output
     System.out.println(Dictionary);
    }
    }
  </code></pre>
  
  <p>This is the sentences file:</p>
  
  <pre><code>1 a rose is a rose 
  2 John chased a cat and the cat chased John
  3 cats are mammals but mammals are not cats
  4 beavers build dams but i know a beaver that does not
  5 my dog chased a cat and the cat attacked my dog
  6 my dog likes cats but my cat dislikes dogs
  7 my dog likes roses but roses dislike my dog
  8 my cat dislikes roses but roses like my cat
  9 red roses are not my favorite roses
  10 my favorite roses are pink roses
  </code></pre>
  
  <p>If I could get some insight on how to intersect the two linked lists(or if there is anything else wrong with my program) I would really greatly appreciate it.  I have been sick for the last week and my professor refuses to help me on what I missed(apparently I am not a serious programmer if I don't come to class when I 'm sick). I really cannot stand the way this professor teaches this class because she doesn't give us any examples of the programs(and the very few she does give us always have errors). She also just gives us algorithms and she's already stated, they are not always correct.  I used to love programming but she has really turned me off on it and all I am trying to do now is get at least a C so I can just switch over to IT. I would really appreciate if someone can help me, I am desperate to just get done with this class and not have to take this professor ever again.</p>
  
  <p>I adding an intersect method but am still receiving all these errors:
  7 errors found:
  File: /export/home/hawkdom2/s0878044/CS503/assignment2/testFile.java  [line: 86]
  Error: /export/home/hawkdom2/s0878044/CS503/assignment2/testFile.java:86: illegal start of expression
  File: /export/home/hawkdom2/s0878044/CS503/assignment2/testFile.java  [line: 86]
  Error: /export/home/hawkdom2/s0878044/CS503/assignment2/testFile.java:86: ';' expected
  File: /export/home/hawkdom2/s0878044/CS503/assignment2/testFile.java  [line: 86]
  Error: /export/home/hawkdom2/s0878044/CS503/assignment2/testFile.java:86: not a statement
  File: /export/home/hawkdom2/s0878044/CS503/assignment2/testFile.java  [line: 86]
  Error: /export/home/hawkdom2/s0878044/CS503/assignment2/testFile.java:86: ';' expected
  File: /export/home/hawkdom2/s0878044/CS503/assignment2/testFile.java  [line: 86]
  Error: /export/home/hawkdom2/s0878044/CS503/assignment2/testFile.java:86: not a statement
  File: /export/home/hawkdom2/s0878044/CS503/assignment2/testFile.java  [line: 86]
  Error: /export/home/hawkdom2/s0878044/CS503/assignment2/testFile.java:86: ';' expected
  File: /export/home/hawkdom2/s0878044/CS503/assignment2/testFile.java  [line: 96]
  Error: /export/home/hawkdom2/s0878044/CS503/assignment2/testFile.java:96: 'else' without 'if'</p>