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| import java.util.ArrayList; | |
| import java.util.Arrays; | |
| import java.util.List; | |
| import java.util.stream.Collectors; | |
| /** | |
| * Answer to http://codereview.stackexchange.com/questions/67804 | |
| * | |
| * Builds all combinations of the given elements. | |
| */ | |
| public class CodeReviewCombinations { | |
| public static <T> List<List<T>> computeCombinationsOriginal(List<List<T>> lists) { | |
| List<List<T>> combinations = new ArrayList<>(); | |
| for (List<T> list : lists) { | |
| List<List<T>> extraColumnCombinations = new ArrayList<>(); | |
| for (T element : list) { | |
| if (combinations.isEmpty()) { | |
| extraColumnCombinations.add(Arrays.asList(element)); | |
| } else { | |
| for (List<T> productList : combinations) { | |
| List<T> newProductList = new ArrayList<>(productList); | |
| newProductList.add(element); | |
| extraColumnCombinations.add(newProductList); | |
| } | |
| } | |
| } | |
| combinations = extraColumnCombinations; | |
| } | |
| return combinations; | |
| } | |
| public static <T> List<List<T>> computeCombinations2(List<List<T>> lists) { | |
| List<List<T>> combinations = Arrays.asList(Arrays.asList()); | |
| for (List<T> list : lists) { | |
| List<List<T>> extraColumnCombinations = new ArrayList<>(); | |
| for (List<T> combination : combinations) { | |
| for (T element : list) { | |
| List<T> newCombination = new ArrayList<>(combination); | |
| newCombination.add(element); | |
| extraColumnCombinations.add(newCombination); | |
| } | |
| } | |
| combinations = extraColumnCombinations; | |
| } | |
| return combinations; | |
| } | |
| public static <T> List<List<T>> appendElements(List<List<T>> combinations, List<T> extraElements) { | |
| return combinations.stream().flatMap(oldCombination | |
| -> extraElements.stream().map(extra -> { | |
| List<T> combinationWithExtra = new ArrayList<>(oldCombination); | |
| combinationWithExtra.add(extra); | |
| return combinationWithExtra; | |
| })) | |
| .collect(Collectors.toList()); | |
| } | |
| public static <T> List<List<T>> computeCombinations3(List<List<T>> lists) { | |
| List<List<T>> currentCombinations = Arrays.asList(Arrays.asList()); | |
| for (List<T> list : lists) { | |
| currentCombinations = appendElements(currentCombinations, list); | |
| } | |
| return currentCombinations; | |
| } | |
| public static void main(String[] args) { | |
| List<Integer> list1 = Arrays.asList(1, 2, 3); | |
| List<Integer> list2 = Arrays.asList(13, 21); | |
| List<Integer> list3 = Arrays.asList(65, 73); | |
| System.out.println("Lists: " + Arrays.asList(list1, list2, list3)); | |
| System.out.println(""); | |
| System.out.println("Original: " + computeCombinationsOriginal(Arrays.asList(list1, list2, list3))); | |
| System.out.println("Shorter: " + computeCombinations2(Arrays.asList(list1, list2, list3))); | |
| System.out.println("Split: " + computeCombinations3(Arrays.asList(list1, list2, list3))); | |
| } | |
| } |
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