** java iterative solution using stack**

public static void preOrder(Node root) {

   Stack<Node> s = new Stack<Node>();
   Node curr = root;
   if(curr==null){
       return;
   }
   while(curr!=null||s.size()>0){
  while(curr!=null){
      s.push(curr);
      System.out.print(curr.data+" ");
      curr=curr.left;   
  }
 curr= s.pop();
  curr=curr.right;
  }



    }