java - permutations of an array (Heap's algorithm)

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import java.lang.reflect.Array; import java.util.ArrayList; import java.util.Arrays; import java.util.Comparator; import java.util.List; import java.util.stream.Collectors; import java.util.stream.Stream; public class Toast { /** * https://en.wikipedia.org/wiki/Permutation * https://en.wikipedia.org/wiki/Heap%27s_algorithm * * @param array * @return */ public static <E> List<E[]> permutations(E[] array) { ArrayList<E[]> list = new ArrayList<>(); permute(array, array.length, list); return list; } static <E> void permute(E[] array, int n, List<E[]> collector) { if(n == 1) collector.add(Arrays.copyOf(array, array.length)); else { for (int i = 0; i < n; i++) { permute(array, n - 1, collector); if (n % 2 == 1) swap(array, 0, n - 1); else swap(array, i, n - 1); } } } }

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