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

给定一个二叉搜索树，要求更改每个节点的值为树中其它比他大的节点值之和。二叉搜索树的特点是中序遍历的结果为一个从小到大的序列。

每个节点的值为其右侧所有节点的和，可以使用中序遍历的方法求解该问题，

class Solution {
    int sum = 0; //全局变量
    public TreeNode convertBST(TreeNode root) {
        if(root == null) return null;
        convertBST( root.right);//先右边
        root.val += sum;
        sum = root.val;
        convertBST( root.left);

        return root;
    }
}