Implementing a nonrecursive preorder traversal method

Question

I need to implement a preorder traversal method. Traversing a binary tree of nodes.

Im trying to figure out a solution for the problem below. I know how to implement such a method, but the problem is that I can't deviate from the rules my teacher gave me. Which makes this exercise substantial harder.

these are the rules:

• the teacher prohibited the use of recursion
• I have to use a stack
• Begin at the root Node

• see the comments in my code for the other restrictions.

  public class Node {
  
   int key;
   String name;
  
   Node leftChild;
   Node rightChild;
  
  public Node(int key, String name){
       this.key = key;
       this.name = name;
  }
  
  // prints information about a certain node
    public void visitStap(){
        System.out.println("Node name : " + this.name );
        System.out.println("Node value : " + this.key + "\n");
     }
  }
  
  
  public void preOrderTraverseTreeNonRecursive(){
      Node current = this.root; // Begin at the root Node
      Stack<Node> theStack = new Stack<Node>(); 
  
      // extra code is allowed here
  
      while(!theStack.empty() || current != null){
          // extra code is allowed here
  
          if(current != null){
              // only 3 lines of code allowed
  
          }else{
              // only 2 lines of code allowed
          }
      }
  }
  

I hope someone could help me with this problem.

Answer 1

I came up with a solution after all.

public void preOrderTraverseTreeNonRecursive(){
    Node current = this.root; // Begin bij de root
    Stack<Node> theStack = new Stack<Node>();

    while(!theStack.empty() || current != null){
        if(current != null){
            // only 3 lines of code allowed
            current.visitStap(); // print current Node information
            theStack.push(current); // push current Node on to the stack
            current = current.leftChild; //  set current node to leftChild
        }else{
            // only 2 lines of code allowed
            current = theStack.pop().rightChild; // pop Node from stack and
                                                 // get its rightChild 

        }
    }
}