flann::hierarchicalClustering get labels

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i am looking for some clustering with dynamic cluster count. i want to use flann::hierarchicalClustering. iam not directly interestet on the cluster itself but on the label of the input data.
how to get the labels? searching for the nearest cluster of each input vector? that seems to be doubled work since this internally should be calculated already
i expected some label output parameter like the bestLabels argument in kmeans.

or is there another (possibly better) way to cluster the data with dynamicly determined cluster count?