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

class Solution:
    def convertBST(self, root):
        """
        :type root: TreeNode
        :rtype: TreeNode
        """
        a = []
        if root is None:
            return root
        def inorder(root):
            if root.left:
                inorder(root.left)
            a.append(root.val)
            if root.right:
                inorder(root.right)
        inorder(root)
        for i in range(len(a)-2,-1,-1):
            a[i] += a[i+1]
        pos = 0
        def change(root):
            nonlocal pos
            if root.left:
                change(root.left)
            root.val = a[pos]
            pos += 1
            if root.right:
                change(root.right)
        change(root)
        return root