```
# 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 convertBST(self, root):
"""
:type root: TreeNode
:rtype: TreeNode
"""
def dfs(root):
if root:
dfs(root.right)
root.val = total[0] = total[0] + root.val
dfs(root.left)
total = [0]
dfs(root)
return root
```