[LeetCode&Python] Problem 860. Convert BST to Greater Tree

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     13

Output: The root of a Greater Tree like this:
             18
            /   \
          20     13

# 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 __init__(self):
        self.total=0
        
    def convertBST(self, root):
        """
        :type root: TreeNode
        :rtype: TreeNode
        """
        if not root:
            return root
        self.convertBST(root.right)
        self.total+=root.val
        root.val=self.total
        self.convertBST(root.left)
        return root
    '''
    def convertBST(self, root):
        """
        :type root: TreeNode
        :rtype: TreeNode
        """
        
        total=0
        stack=[]
        node=root
        while node is not None or stack:
            while node is not None:
                stack.append(node)
                node=node.right
            node=stack.pop()
            total+=node.val
            node.val=total
            node=node.left
        return root