Convert a Binary Tree to Doubly Linked List

/*

Convert a binary search tree to doubly linked list with in-order traversal.

Example
Given a binary search tree:

    4
   / \
  2   5
 / \
1   3
return 1<->2<->3<->4<->5

Tags Expand 
Linked List
*/

/*
    Thoughts:
    Inorder with 1 stack: peek add left till end, pop and add, then push right node.
    
    Everytime when pop out a node and add, make it a new boubllistnode
        dNode.next = curr
        curr.pre = dNode.next
        dNode = dNode.next
        
    boarder case: if null, return a null.
*/
public class Solution {
    public DoublyListNode bstToDoublyList(TreeNode root) {  
        if (root == null) {
            return null;
        }
        //Init stack
        Stack<TreeNode> stack = new Stack<TreeNode>();
        TreeNode node = root;    
        stack.push(node);
        //Create DoublyListNode header
        DoublyListNode dummy = new DoublyListNode(0);
        DoublyListNode dNode = dummy;
        
            
        while(!stack.isEmpty()) {
            while (node != null && node.left != null) {
                stack.push(node.left);
                node = node.left;
            }
            //add node
            node = stack.pop();
            DoublyListNode curr = new DoublyListNode(node.val);
            dNode.next = curr;
            curr.prev = dNode;
            dNode = dNode.next;
            
            //check right node and add to stack
            node = node.right;
            if (node != null) {
                stack.push(node);
            }  
        }
        
        return dummy.next;
        
    }
}