思路：root.val += root.right     root.left += root.val

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

class solution:
    def __init__(self):
        self.sum = 0
    def convertBST(self, root):
        if not root:
            return None
        self.convertBST(root.right)
        self.sum += root.val
        root.val = self.sum
        self.convertBST(root.left)
        return root
 

右孩子加到root.val上給root.val 

root.val加到左孩子給左孩子

用self.sum存放當前遍歷過的節點值之和