code: public List<E> next() {
          if (numLeft.equals(total)) {
              numLeft = numLeft.subtract(BigInteger.ONE);
              return items;
          }
  
          int temp;
  
          // Find largest index j with a[j] < a[j+1]
          int j = a.length - 2;
          while (a[j] > a[j + 1]) {
              j--;
          }
  
          // Find index k such that a[k] is smallest integer
          // greater than a[j] to the right of a[j]
          int k = a.length - 1;
          while (a[j] > a[k]) {
              k--;
          }
  
          // Interchange a[j] and a[k]
          temp = a[k];
          a[k] = a[j];
          a[j] = temp;
  
          // Put tail end of permutation after jth position in increasing order
          int r = a.length - 1;
          int s = j + 1;
  
          while (r > s) {
              temp = a[s];
              a[s] = a[r];
              a[r] = temp;
              r--;
              s++;
          }
  
          numLeft = numLeft.subtract(BigInteger.ONE);
          List<E> ans = new ArrayList<E>(a.length);
          for (int index : a) {
              ans.add(items.get(index));
          }
          return ans;
      }
docs: Generate next permutation (algorithm from Rosen p. 284)
  
  @return the items permuted