leetcode 538. Convert BST to Greater Tree（BST转变为较大树）

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:

把BST每个节点的值都换成 节点值+比它大的所有节点值的和

思路：
观察了一下，发现是按照右子树，root，左子树的顺序求和加到root.val上的，可以写一个按这种顺序遍历求和

class Solution {
    int sum = 0;
    public TreeNode convertBST(TreeNode root) {
        rightTraversal(root);
        return root;
    }
    
    void rightTraversal(TreeNode root) {
        if(root == null) return;
        rightTraversal(root.right);
        sum += root.val;
        root.val = sum;
        rightTraversal(root.left);
    }
}