1373. Maximum Sum BST in Binary Tree

https://leetcode.com/problems/maximum-sum-bst-in-binary-tree

Description

Given a binary tree root, return the maximum sum of all keys of any sub-tree which is also a Binary Search Tree (BST).

Assume a BST is defined as follows:

• 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 = [1,4,3,2,4,2,5,null,null,null,null,null,null,4,6]
**Output:** 20
**Explanation:** Maximum sum in a valid Binary search tree is obtained in root node with key equal to 3.

Example 2:

**Input:** root = [4,3,null,1,2]
**Output:** 2
**Explanation:** Maximum sum in a valid Binary search tree is obtained in a single root node with key equal to 2.

Example 3:

**Input:** root = [-4,-2,-5]
**Output:** 0
**Explanation:** All values are negatives. Return an empty BST.

Example 4:

**Input:** root = [2,1,3]
**Output:** 6

Example 5:

**Input:** root = [5,4,8,3,null,6,3]
**Output:** 7

Constraints:

• The number of nodes in the tree is in the range [1, 4 * 104].

• -4 * 104 <= Node.val <= 4 * 104

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