code: public static int[] concatArrays(int[] a, int[] b) {
          final int alen = a.length;
          final int blen = b.length;
          if (alen == 0) {
              return b;
          }
          if (blen == 0) {
              return a;
          }
          final int[] result = new int[alen + blen];
          arraycopy(a, 0, result, 0, alen);
          arraycopy(b, 0, result, alen, blen);
          return result;
      }
docs: Concatenates the two arrays {@code a} and {@code b} and returns the
  result.
  
  @param a {@code int[]} a
  @param b {@code int[]} b
  @return {@code int[]} concatenation