/** Gets all permutations of a given word. */
public ArrayList<String> getPermutations() {
ArrayList<String> result = new ArrayList<String>();
// The empty string has a single permutation: itself
if (word.length() == 0) {
result.add(word);
return result;
}
// Loop through all character positions
for (int i = 0; i < word.length(); i++) {
// Form a simpler word by removing the ith character
String shorterWord = word.substring(0, i) + word.substring(i + 1);
// Generate all permutations of the simpler word
Permutations shorterPermutations = new Permutations(shorterWord);
ArrayList<String> shorterWordPermutations = shorterPermutations.getPermutations();
// Add the removed character to the front of
// each permutation of the simpler word,
for (String s : shorterWordPermutations) {
result.add(word.charAt(i) + s);
}
}
// Return all permutations
return result;
}
@Test
public void testSimple() throws Exception {
final String s = "ab";
final Set<String> expected = new HashSet<>(Arrays.asList("ab", "ba"));
assertEquals(expected, Permutations.perms(s));
}