動機

順著走就好

Problem

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 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.

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

 

Example 1:

Input: root = [4,1,6,0,2,5,7,Leetcode,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]

Example 3:

Input: root = [1,0,2]Output: [3,3,2]

Example 4:

Input: root = [3,2,4,1]Output: [7,9,4,10]

 

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.
  • root is guaranteed to be a valid binary search tree.

Sol

class Solution:
    def bstToGst(self, root: TreeNode) -> TreeNode:
        def post(root, cnt=0):
            if not root:
                return cnt
            else:
                right = post(root.right, cnt)
                root.val += right
                return post(root.left, root.val)
        post(root)
        return root