I use modified inorder traversal since it is a BST.

right -> root -> left

```
# 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
"""
# right -> center -> left traverse, accumulate node value
self.accum = 0
self._inorder(root)
return root
def _inorder(self, node):
if not node: return
self._inorder(node.right)
tmp = node.val
node.val = node.val + self.accum
self.accum = self.accum + tmp
self._inorder(node.left)
return
```