A permutation \pi of some \{1,2,…, n\} is a rearrangement of this list. There are n! different permutations of this set. A permutation is an ORDERED arrangement of objects. permutation with indistinct objects What if you want to order a set with sub-set of indistinct objects? Like, for instance, how many ways are there to order:
\begin{equation} 10100 \end{equation}
For every permutation of 1 in this set, there are two copies being overcounted. Let there are n objects. n_1 objects are the indistinct, n_2 objects are indistinct, … n_{r} objects are the same. The number of permutations are:
\begin{equation} \frac{n!}{{n_1}!{n_2}! \dots {n_r}!} \end{equation}
You can use iterators to give you permutations.