538.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:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def __init__(self):
        self.total = 0

    def convertBST(self, root):
        """
        :type root: TreeNode
        :rtype: TreeNode
        """
        if root is None:
            return root

        self.convertBST(root.right)
        self.total += root.val
        root.val = self.total
        self.convertBST(root.left)

        return root