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

反向中序遍历

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