/** I literally studied how permutation is implemented in Python and decided to give it a try in Java and amazing enough, it works just fine. The advantage of this algorithm is that it allows you to choose the the size of the permuted values. For those who are familiar with the common math expression 'n choose k' understand it just fine. The common algorithm I found on net only implements 'n choose n'. where n is the size of the list and k is the amount to choose in each choice. Check it out and don't forget the like or dislike button and or a comment. */ // we need only two imports import java.util.ArrayList; import java.util.List; public class Permutation { private List> permuted = new ArrayList<>(); public static void main(String[] args) { Permutation permutation = new Permutation<>(); // test with a list object with Letters R, A and M List list = new ArrayList<>(); list.add("R"); list.add("A"); list.add("M"); // testing with n=3 and k=2 System.out.println(permutation.permute(list, 2)); } public List> permute(List list) { // returns n choose n return permute(list, list.size()); } public List> permute(List list, int r) { // n choose k where k = r List pool = new ArrayList<>(list); int n = list.size(); if (r > n) { throw new IllegalArgumentException("argument value cannot be greater than the size " + "of the argument list."); } List indices = new ArrayList<>(); List cycles = new ArrayList<>(); List reversed = new ArrayList<>(); List> results = new ArrayList<>(); List res = new ArrayList<>(); for (int i = 0; i < n; i++) { indices.add(i); } for (int i = n; i > n - r; i--) { cycles.add(i); } for (int i = r; i > 0; i--) { reversed.add(i - 1); } for (int i = 0; i < r; i++) { res.add(pool.get(i)); } results.add(new ArrayList<>(res)); boolean broken = false; while (n > 0) { for (int i : reversed) { cycles.set(i, cycles.get(i) - 1); if (cycles.get(i) == 0) { List temp = new ArrayList<>(); temp.addAll(indices.subList(0, i)); temp.addAll(indices.subList(i + 1, n)); temp.addAll(indices.subList(i, i + 1)); indices = new ArrayList<>(temp); cycles.set(i, n - i); broken = false; } else { int j = cycles.get(i); int x = indices.get(i); indices.set(i, indices.get(n - j)); indices.set(n - j, x); res = new ArrayList<>(); for (int k = 0; k < r; k++) { res.add(pool.get(indices.get(k))); } results.add(new ArrayList<>(res)); broken = true; break; } } if (!broken) { break; } } return results; } }

Python's Permutation in Java