code: public static <T> List<T> distinct(final Collection<? extends T> c) {
          if (N.isNullOrEmpty(c)) {
              return new ArrayList<>();
          }
  
          return distinct(c, 0, c.size());
      }
docs: Mostly it's designed for one-step operation to complete the operation in one step.
  <code>java.util.stream.Stream</code> is preferred for multiple phases operation.
  
  @param c
  @return