* requireNonNull is safe because we don't clear nextPermutation until we're done calling this
       * method.
       */
      requireNonNull(nextPermutation);

      int l = findNextL(j);
      Collections.swap(nextPermutation, j, l);
      int n = nextPermutation.size();
      Collections.reverse(nextPermutation.subList(j + 1, n));
    }

    int findNextJ() {
      /*

       * requireNonNull is safe because we don't clear nextPermutation until we're done calling this
       * method.
       */
      requireNonNull(nextPermutation);

      int l = findNextL(j);
      Collections.swap(nextPermutation, j, l);
      int n = nextPermutation.size();
      Collections.reverse(nextPermutation.subList(j + 1, n));
    }

    int findNextJ() {
      /*