538. Convert BST to Greater Tree

Description https://leetcode.com/problems/convert-bst-to-greater-tree/description/
Solution:
Recursion. Use inverse in-order recursive traversal and use a variable to store the sum greater than the current node.
class Solution {
private:
    int gSum = 0;
public:
    void invInOrder(TreeNode* node){
        if(!node) return;
        if(node->right) invInOrder(node->right);
        gSum+=node->val;
        node->val = gSum;
        if(node->left) invInOrder(node->left);
    }
    TreeNode* convertBST(TreeNode* root) {
        invInOrder(root);
        return root;
    }
};