給定一個二叉搜索樹（Binary Search Tree），把它轉換成為累加樹（Greater Tree)，使得每個節點的值是原來的節點值加上所有大於它的節點值之和。
例如：
輸入: 二叉搜索樹:
5
/ \
2 13
輸出: 轉換為累加樹:
18
/ \
20 13
詳見：https://leetcode.com/problems/convert-bst-to-greater-tree/description/

C++：

方法一：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* convertBST(TreeNode* root) 
    {
        int sum = 0;
        helper(root, sum);
        return root;
    }
    void helper(TreeNode*& node, int& sum)
    {
        if (!node)
        {
            return;
        }
        helper(node->right, sum);
        node->val += sum;
        sum = node->val;
        helper(node->left, sum);
    }
};

方法二：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* convertBST(TreeNode* root) {
        if (!root)
        {
            return nullptr;
        }
        int sum = 0;
        stack<TreeNode*> stk;
        TreeNode *p = root;
        while (p || !stk.empty())
        {
            while (p)
            {
                stk.push(p);
                p = p->right;
            }
            p = stk.top(); 
            stk.pop();
            p->val += sum;
            sum = p->val;
            p = p->left;
        }
        return root;
    }
};

參考：http://www.cnblogs.com/grandyang/p/6591526.html

538 Convert BST to Greater Tree 把二叉搜索樹轉換為累加樹