/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode() : val(0), left(nullptr), right(nullptr) {}
* TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
* TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
int pre = 0;
TreeNode* convertBST(TreeNode* root) {
// 反中序遍历
if (!root) return nullptr;
convertBST(root->right);
pre += root->val;
root->val = pre;
convertBST(root->left);
return root;
}
};