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class Solution {
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  private int sum = 0;
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  public int rangeSumBST(TreeNode root, int low, int high) {
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    dfs(root, low, high);
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    return sum;
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  }
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  public void dfs(TreeNode root, int low, int high) {
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    if (root == null) return;
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    if (root.val < low) dfs(root.right, low, high);
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    else if (root.val > high) dfs(root.left, low, high);
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    if (root.val >= low && root.val <= high) {
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      sum += root.val;
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      dfs(root.left, low, high);
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      dfs(root.right, low, high);
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    }
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  }
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}