code: public static <T> List<T> repeatEach(final Collection<T> c, final int n) {
          N.checkArgNotNegative(n, "n");
  
          if (n == 0 || isNullOrEmpty(c)) {
              return new ArrayList<T>();
          }
  
          final List<T> result = new ArrayList<>(c.size() * n);
  
          for (T e : c) {
              for (int i = 0; i < n; i++) {
                  result.add(e);
              }
          }
  
          return result;
      }
docs: Repeats the elements in the specified Collection one by one.
  
  <pre>
  <code>
  Seq.nRepeat(N.asList(1, 2, 3), 2) => [1, 1, 2, 2, 3, 3]
  </code>
  </pre>
  
  @param c
  @param n
  @return