BST to double linkedlist in inorder traversal sequence
Follow Up: 如果这个BST自带prev, next, 给一个value，插进去这个node，并补全left，right，prev，next

 class TreeNode {

  public int val;

  TreeNode left;

  TreeNode right;

 }

 TreeNode newRoot

 TreeNode tree2Dll(TreeNode root) {

   TreeNode pre = null;

   TreeNode newRoot = null;

   helper(root, pre);

   return newRoot;

 }

 public void helper(TreeNode cur, TreeNode pre) {

   if (cur == null) {

     return;

   }

   helper(cur.left, pre);

   cur.left = pre;

   if (pre == null) {

      newRoot = cur;

   }

   else pre.right = cur;

   pre = cur;

   helper(cur.right, pre, newRoot);

 }