Leetcode 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.

題目大意是： 把一個樹變成另外一個樹，那個樹每一個節點是原來的樹所有比它大的節點之和。

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      

1、右節點不變（因為BST的性質），目前節點加（2+13），左節點加（5+13）.

2、用歸遞，inorder traverse tree，但是先右再左

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