538. 把二叉搜索樹轉換爲累加樹（樹）

遍歷順序，右根左

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

        return root;
    }
}