@Override
  public boolean canMerge(NumDistinctValueEstimator o) {
    return o instanceof HyperLogLog;
  }
Converts sparse to dense hll register.
@param sparseRegister
         - sparse register to be converted
@return converted dense register
standalone-metastore/metastore-server/src/main/java/org/apache/hadoop/hive/common/ndv/hll/HyperLogLog.java