Binary Search Tree to Greater Sum Tree LeetCode solution in C++

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