public T find(T element) {
    if (!map.containsKey(element)) {
      map.put(element, new Entry<>());
      return element;
    }
    return findInternal(element);
  }
@return <tt>true</tt> if the specified equivalence is new
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/relational/utils/DisjointSet.java