題目：

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.

思路：

每個結點值加上所有比它大的結點值總和當作新的結點值。

初始化sum值 = 0

因為inorder traversal （left->root->right）結果是non decreasing 遞增數列，那麼逆過來right->root->left

程式碼：

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