1
var rangeSumBST = function (root, low, high) {
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  let sum = 0;
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  const summer = (root) => {
4
    if (!root) {
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      return;
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    }
7

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    sum = root.val >= low && root.val <= high ? root.val + sum : sum;
9

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    summer(root.left);
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    summer(root.right);
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  };
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  summer(root);
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  return sum;
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};