LeetCode 538 Convert BST to Greater Tree

给予树：

     5
   /   \
  2     13

返回树:
     18
    /   \
  20     13

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int sum = 0;
    public TreeNode convertBST(TreeNode root) {
        if (root == null) {
            return null;
        }

        convertBST(root.right);

        root.val += sum;
        sum = root.val;

        convertBST(root.left);
        return root;
    }
}