0538. Convert BST to Greater Tree

https://leetcode.com/problems/convert-bst-to-greater-tree

Description

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 1038: https://leetcode.com/problems/binary-search-tree-to-greater-sum-tree/

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]

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 [0, 104].

• -104 <= Node.val <= 104

• All the values in the tree are unique.

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

ac

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int sum = 0;

    public TreeNode convertBST(TreeNode root) {
        if (root == null) return root;
        convertBST(root.right);
        sum += root.val;
        root.val = sum;
        convertBST(root.left);
        return root;
    }
}

/*
1) get total sum; 2) in-order traversal, total sum - prev sum;
update from right to left, add each node.val to sum, update node.val = sum
*/