538 Convert BST to Greater Tree 把二叉搜索树转换为累加树

给定一个二叉搜索树（Binary Search Tree），把它转换成为累加树（Greater Tree)，使得每个节点的值是原来的节点值加上所有大于它的节点值之和。
例如：
输入: 二叉搜索树:
              5
            /   \
           2     13
输出: 转换为累加树:
             18
            /   \
          20     13
详见：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;
    }
};

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