LeetCode: Convert BST to Greater Tree

利用好BST的特性，解起来并不难

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    private 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;
    }
}