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

 二叉查找树（Binary Search Tree），（又：二叉搜索树，二叉排序树）

它或者是一棵空树，

或者是具有下列性质的二叉树：

             若它的左子树不空，则左子树上所有结点的值均小于它的根结点的值；

             若它的右子树不空，则右子树上所有结点的值均大于它的根结点的值；

             它的左、右子树也分别为二叉排序树。

  public TreeNode ConvertBST(TreeNode root)
        {
            if (root == null)
            {
                return null;
            }

            int sum = 0;
            Chuck(root,ref sum);
            return root;
        }

        private void Chuck(TreeNode node, ref int sum)
        {
            if (node == null)
            {
                return;
            }

            Chuck(node.right, ref sum);
            sum = sum + node.val;
            node.val = sum;
            Chuck(node.left, ref sum);
        }