So I'm using the classic algorithm for permutations which is
private static void permutation(String prefix, String str) { int n = str.length(); if (n == 0) System.out.println(prefix); else { for (int i = 0; i < n; i++) permutation(prefix + str.charAt(i), str.substring(0, i) + str.substring(i+1, n)); } } The only thing I can't figure out how to do is how to store the permutations into an array instead of just printing them out. Any help is appreciated.
Try this method. This will return list of permutations
private static List<String> permutation(String prefix, String str) { List<String> permutations = new ArrayList<>(); int n = str.length(); if (n == 0) { permutations.add(prefix); } else { for (int i = 0; i < n; i++) permutations.addAll(permutation(prefix + str.charAt(i), str.substring(0, i) + str.substring(i + 1, n))); } return permutations; } Using Arrays. JAVA 8 needed
private static String[] permutation(String prefix, String str) { String[] permutation = new String[]{}; int n = str.length(); if (n == 0) { permutation = new String[]{prefix}; } else { for (int i = 0; i < n; i++) permutation = Stream.concat(Arrays.stream(permutation(prefix + str.charAt(i), str.substring(0, i) + str.substring(i + 1, n))), Arrays.stream(permutation)).toArray(String[]::new); } return permutation; } You could also just pass a reference of the list of permutations to the permutation method itself. This has the advantage of only creating one list.
private static List<String> permutation(String str) { List<String> perms = new ArrayList<>(); permutation("", str, perms); return perms; } private static void permutation(String prefix, String str, List<String> perms) { int n = str.length(); if ( n == 0 ) { perms.add(prefix); } else { for ( int i = 0; i < n; i++ ) permutation(prefix + str.charAt(i), str.substring(0, i) + str.substring(i+1,n), perms); } } List<String> perms = permutation("abcd"); 