# Definition for a binary tree node.
# class TreeNode:
#     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
        """
        self.help(root, 0)
        return root
    
    def help(self, root, temp):
        if not root:
            return temp
        right = self.help(root.right, temp)
        temp = root.val = root.val+right
        left = self.help(root.left, temp)
        return left