问题
I am trying to generate all permutations of a List<List<String>> but I am not getting unique permutations.
My List<List<String>> looks something like
[
[Test, Test 1, Test 2],
[Apple, Sandwich, Banana],
[Cake, Ice Cream, Fruit]
]
I am trying to get every combination possible for each List<String within the parent List<String>. So, for example, the first instance should have:
[Test, Test 1, Test 2]
[Test, Test 2, Test 1]
[Test 2, Test, Test 1]
[Test 2, Test 1, Test]
[Test 1, Test, Test 2]
[Test 1, Test 2, Test]
Right now, my attempt will just iterate and repeat the elements without changing the order so [Test, Test 1, Test 2] will just be repeated for the size of the parent List<String>. This is my approach. Any help is appreciated:
List<List<String>> allPerms = parentList.stream().map(line -> parentList.stream()).flatMap(l -> l.filter(j -> !j.equals(l))).collect(Collectors.toList());
回答1:
Use CollectionUtils.permutations
public static void showAllPermuteByCommons(List<String> list) {
CollectionUtils.permutations(list) //
.stream() //
.forEach(System.out::println);
}
Or backtrack
public static void permute(List<String> list, int left, int right) {
if (left == right) {
System.out.println(Arrays.toString(list.toArray()));
return;
}
for (int j = left; j <= right; j++) {
Collections.swap(list, left, j);
permute(list, left + 1, right);
Collections.swap(list, left, j);
}
}
public static void showAllPermute(List<String> list) {
int size = list.size();
permute(list, 0, size - 1);
}
Test
List<List<String>> list = new ArrayList<>();
list.add(Arrays.asList("Test", "Test 1", "Test 2"));
list.add(Arrays.asList("Apple", "Sandwich", "Banana"));
list.add(Arrays.asList("Cake", "Ice Cream", "Fruit"));
// list.forEach(t -> showAllPermuteByCommons(t));
list.forEach(t -> showAllPermute(t));
Ouput
[Test, Test 1, Test 2]
[Test, Test 2, Test 1]
[Test 2, Test, Test 1]
[Test 2, Test 1, Test]
[Test 1, Test 2, Test]
[Test 1, Test, Test 2]
[Apple, Sandwich, Banana]
[Apple, Banana, Sandwich]
[Banana, Apple, Sandwich]
[Banana, Sandwich, Apple]
[Sandwich, Banana, Apple]
[Sandwich, Apple, Banana]
[Cake, Ice Cream, Fruit]
[Cake, Fruit, Ice Cream]
[Fruit, Cake, Ice Cream]
[Fruit, Ice Cream, Cake]
[Ice Cream, Fruit, Cake]
[Ice Cream, Cake, Fruit]
来源：https://stackoverflow.com/questions/58314196/how-to-generate-all-permutations-of-a-listliststring