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class Solution {
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  public TreeNode trimBST(TreeNode root, int low, int high) {
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    if (root == null) return root;
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    while (root.val < low || root.val > high) {
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      root = root.val < low ? root.right : root.left;
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      if (root == null) return root;
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    }
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    root.left = trimBST(root.left, low, high);
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    root.right = trimBST(root.right, low, high);
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    return root;
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  }
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}