题目如下：

Given a Binary Search Tree (BST), convert it to a Greater Tree such that every key of the original BST is changed to the original key plus sum of all keys greater than the original key in BST.

Example:

Input: The root of a Binary Search Tree like this:              5            /   \           2     13Output: The root of a Greater Tree like this:             18            /   \          20     13

解题思路：一时卡壳了，只想出了一个很挫的方法，把所有节点的值都取出来，然后找出每个节点的所有比其大的值的和，这个值就是这个节点的值要加上的大小，最后再遍历一次树修改节点。

代码如下：

# Definition for a binary tree node.# class TreeNode(object):#     def __init__(self, x):#         self.val = x#         self.left = None#         self.right = Noneclass Solution(object):    l = []    dic = {}    def traverse(self,node,mode):        if node == None:            return        if mode == 1:            self.l.append(node.val)        else:            node.val += self.dic[node.val]        if node.left != None:            self.traverse(node.left,mode)        if node.right != None:            self.traverse(node.right,mode)    def convertBST(self, root):        """        :type root: TreeNode        :rtype: TreeNode        """        self.l = []        self.dic = {}        self.traverse(root,1)        ul = sorted(list(set(self.l)))        total = 0        for i in ul[::-1]:            self.dic[i] = total            total += i        self.traverse(root, 0)        return root