LeetCode 538. 把二叉搜索树转换为累加树（逆中序 根右左）

例如：

输入: 二叉搜索树:
              5
            /   \
           2     13

输出: 转换为累加树:
             18
            /   \
          20     13

class Solution {
public:
    TreeNode* convertBST(TreeNode* root) {
        stack<TreeNode*> stk;
        int prevSum = 0;
        TreeNode* rootcopy = root;
        while(root || !stk.empty())
        {
        	while(root)
        	{
        		stk.push(root);
        		root = root->right;
        	}
        	root = stk.top();
        	prevSum += root->val;
        	root->val = prevSum;
        	root = root->left;
        	stk.pop();
        }
        return rootcopy;
    }
};

class Solution {
    int sum = 0;
public:
    TreeNode* convertBST(TreeNode* root) {
        if(!root) return NULL;
        convertBST(root->right);
        sum += root->val;
        root->val = sum;
        convertBST(root->left);
        return root;
    }
};