/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 * Definition for Doubly-ListNode.
 * public class DoublyListNode {
 *     int val;
 *     DoublyListNode next, prev;
 *     DoublyListNode(int val) {
 *         this.val = val;
 *         this.next = this.prev = null;
 *     }
 * }
 */
public class Solution {
    private DoublyListNode head = null;
    private DoublyListNode tail = null;
    /**
     * @param root: The root of tree
     * @return: the head of doubly list node
     */
    public DoublyListNode bstToDoublyList(TreeNode root) {
        // Write your code here
        if (null == root) {
            return null;
        }
        bstToDoublyList(root.left);

        DoublyListNode node = new DoublyListNode(root.val);
        if (null == head) {
            head = node;
            tail = node;
        } else {
            tail.next = node;
            node.prev = tail;
            tail = node;
        }

        bstToDoublyList(root.right);

        return head;
    }
}