Convert BST to Greater Tree.py

'''
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 convertBST(self, root):
        """
        :type root: TreeNode
        :rtype: TreeNode
        """
        
        self.get(root, 0)
        
        return root
    
    def get(self, root, total):
        if not root:
            return 0
        
        if root.right:
            total = self.get(root.right, total)
        total += root.val
        root.val = total
        if root.left:
            total = self.get(root.left, total)
        return total