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public static TreeNode trimBST(TreeNode cur, int low, int high){
    // 递归终止条件：遇到空节点直接返回
    if(cur == null){
        return null;
    }
    
    // 情况1：当前节点太小，答案一定在右子树
    if(cur.val < low){
        return trimBST(cur.right, low, high);
    }
    
    // 情况2：当前节点太大，答案一定在左子树
    if(cur.val > high){
        return trimBST(cur.left, low, high);
    }
    
    // 情况3：当前节点在范围内，保留它，并递归修剪左右子树
    cur.left = trimBST(cur.left, low, high);
    cur.right = trimBST(cur.right, low, high);
    
    return cur;
}