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class Solution {
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 public:
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  int solve(TreeNode *root, int low, int high) {
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    if (root == NULL) return 0;
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    int sum = 0;
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    if (low <= root->val && root->val <= high) {
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      sum = root->val;
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    }
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    return sum + solve(root->left, low, high) + solve(root->right, low, high);
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  }
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  int rangeSumBST(TreeNode *root, int low, int high) {
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    return solve(root, low, high);
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  }
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};