刚开始的思路，先遍历求和，在通过先序遍历一个个求解，但是因为无法记录和的值，只能告吹。
错误解法(记录下来是因为熟悉bfs，以及python中没有和c++中int &一样的语法的问题，需要牢记。)：
class Solution:
def bstToGst(self, root: TreeNode) -> TreeNode:
s = self.bst(root)
s1 = 0
self.Inorder(root,s,s1)
return root
def Inorder(self,root,s,s1):
if root == None:
return
self.Inorder( root.left,s,s1)
s1 = s1 + root.val
root.val = s -s1+ root.val
self.Inorder(root.right,s,s1)
def bst(self, root: TreeNode) -> TreeNode:
tree_sum = 0
if root is None:
return tree_sum
queue = [root]
while queue:
cur_node = queue.pop(0)
tree_sum += cur_node.val
if cur_node.left is not None:
queue.append(cur_node.left)
if cur_node.right is not None:
queue.append(cur_node.right)
return tree_sum
