Postfix in expression tree

Question

Hello,
I am trying to put this postfix stack :
Stack postfix = [2, 3, +]
into an expression tree.
This is the code i have written to make a tree:
     private void makeTree(Stack postfix)     {          System.out.println("Creating tree in makeTree() " + postfix);          Stack tempStack = new Stack();                    if (!postfix.isEmpty())          {               String s = (String) postfix.pop();               System.out.println("Character popped from postfix stack " + s);                              if (isOperator(s) == false)               {                    //store popped number on tempStack                    tempStack.push(s);                    System.out.println("Pushed into tempStack " + tempStack);               }               else //popped element is an operator               {                    //create the tree with the nodes                    System.out.println("Popped element is an operator: " + s);                    System.out.println(tempStack);                    if(!tempStack.isEmpty())                    {                         System.out.println("Stack is not empty!");                         Node right = (Node) tempStack.pop();                         Node left = (Node) tempStack.pop();                         tempStack.push(new Node (s, left, right));                    }                                   }               root = (Node) tempStack.pop();          }     }//end of method makeTree()
When i check the tempStack with a print statement, it says that the stack is empty, thats why it doesn't access this part of the code:
if(!tempStack.isEmpty())                    {                         System.out.println("Stack is not empty!");                         Node right = (Node) tempStack.pop();                         Node left = (Node) tempStack.pop();                         tempStack.push(new Node (s, left, right));                    }
I have put so many print statements in this code and other methods, i just can't find out what doesn't work.
I would appreciate any help.
thanks, jen

Answer 1

I don't know... i just can't get this to work. This is what i have done with the code now:
import java.util.*;
public class ExpressionTree
     static String node;
     private class Node
          public String data;
          public Node left;
          public Node right;
          public Node (String data)
               this.data = data;
          public Node (String data, Node left, Node right)
               this(data);
               this.left = left;
               this.right = right;
     }//end of class Node
     private Node root;
     public ExpressionTree(Stack postfix)
        System.out.println("Constructor Expression Tree created");
          buildTree(postfix);
    public String buildTree(Stack s)
           String e = (String) s.pop();
           if (isOperator(e) == true)        
            String right = buildTree(s);
                  String left = buildTree(s);
            Node(e, left, right);
            node = e + left + right;
                  return node;
        else // operand assumed
             Node(e);
             node = e;
             return node;
}the error message i get is:
java:42: cannot resolve symbol
[javac] symbol : method Node (java.lang.String,java.lang.String,java.lang.String)
[javac] location: class ExpressionTree
[javac]           Node(e, left, right);
[javac] ^
I don't know whats wrong with this, because i am passing 3 strings to the Node method.
Thanks for your help!.