Increasing Order Search Tree

Problem Statement:
Given a binary search tree, rearrange the tree in in-order so that the leftmost node in the tree is now the root of the tree, 
and every node has no left child and only 1 right child.
Example 1:
Input: [5,3,6,2,4,null,8,1,null,null,null,7,9]

       5
      / \
    3    6
   / \    \
  2   4    8
 /        / \ 
1        7   9

Output: [1,null,2,null,3,null,4,null,5,null,6,null,7,null,8,null,9]

 1
  \
   2
    \
     3
      \
       4
        \
         5
          \
           6
            \
             7
              \
               8
                \
                 9  


Solution:
# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def increasingBST(self, root):
        li=[]
        def inorder(root,li):
            if root:
                inorder(root.left,li)
                li.append(root.val)
                inorder(root.right,li)
            return li
        inorder(root,li)
        ans=cur=TreeNode(li[0])
        for i in range(1,len(li)):
            cur.right=TreeNode(li[i])
            cur=cur.right
        return ans