538. Convert BST to Greater Tree

Given 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.

Example:

Input: The root of a Binary Search Tree like this:
              5
            /   
           2     13

Output: The root of a Greater Tree like this:
             18
            /   
          20     13

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

inorder的bst‘是increasing，那reverse inorder右根左不就是decreasing了吗

设一个全局变量sum，到根了就更新root.val和sum