538. Convert BST to Greater Tree

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.

Example:

Input: The root of a Binary Search Tree like this:
              5
            /   \
           2     13

Output: The root of a Greater Tree like this:
             18
            /   \
          20     13

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将搜索二叉树按节点的值从大到小累加。累加过程中，累加的值存在节点中。这种按顺序的操作联想到了中序遍历，可以看做是从后向前的中序遍历。

代码：

class Solution
{
public:
	TreeNode* convertBST(TreeNode* root)
	{
		inorder(root);
		return root;
	}
private:
	void inorder(TreeNode* root)
	{
		if(root)
		{
			inorder(root->right);
			root->val += sum;
			sum = root->val;
			inorder(root->left);
		}
	}

	int sum = 0;
};