1038. Binary Search Tree to Greater Sum Tree

Medium

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Given the root of 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 the sum of all keys greater than the original key in BST.

As a reminder, a binary search tree is a tree that satisfies these constraints:

• The left subtree of a node contains only nodes with keys less than the node's key.

• The right subtree of a node contains only nodes with keys greater than the node's key.

• Both the left and right subtrees must also be binary search trees.

Example 1:

Input: root = [4,1,6,0,2,5,7,null,null,null,3,null,null,null,8]
Output: [30,36,21,36,35,26,15,null,null,null,33,null,null,null,8]

Example 2:

Input: root = [0,null,1]
Output: [1,null,1]

Constraints:

• The number of nodes in the tree is in the range [1, 100].

• 0 <= Node.val <= 100

• All the values in the tree are unique.

Note: This question is the same as 538: https://leetcode.com/problems/convert-bst-to-greater-tree/

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def __init__(self):
        self.total = 0
    def bstToGst(self, root: TreeNode) -> TreeNode:
        if root is not None:
            self.bstToGst(root.right)
            self.total += root.val
            root.val = self.total
            self.bstToGst(root.left)
        return root