```
# 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 inorder(self, root, run_sum):
if not root:
return run_sum
root.val += self.inorder(root.right, run_sum)
return self.inorder(root.left, root.val)
def convertBST(self, root):
"""
:type root: TreeNode
:rtype: TreeNode
"""
self.inorder(root, 0)
return root
```