Convert BST to Greater Tree

Question

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

Tags

• Tree

Thought

In-order traversal the tree from right to left.

Code

# 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):
    nodeValues = 0
    def helper(self, root):
        if root is None:
            return
        if root.right is not None:
            self.convertBST(root.right)
        self.nodeValues += root.val
        root.val = self.nodeValues
        if root.left is not None:
            self.convertBST(root.left)

    def convertBST(self, root):
        """
        Tree traversal from right to left
        :type root: TreeNode
        :rtype: TreeNode
        """
        self.helper(root)
        return root