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

大致意思是每個節點加上比這個節點大的所有值，生成一個新的數

中序遍歷即可

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

538. Convert BST to Greater Tree