Convert BST to Greater Tree LeetCode solution in C++

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class Solution {
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 public:
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  int val = 0;
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  TreeNode *convertBST(TreeNode *root) {
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    if (root) {
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      convertBST(root->right); // traverse right sub-tree
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      val += root->val;        // add val
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      root->val = val;         // update val
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      convertBST(root->left);  // traverse left sub-tree
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    }
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    return root;
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  }
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};