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
题目含义：：给定一棵二叉搜索树，将它转化为更大的二叉树，规则是：结点的值 = 该结点的值 + 所有比它大的结点的值的和。
思路：从右向左依次累加求出总和sum，然后将sum付给当前节点值

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