鉴于未知数量的列表,每个列表的长度未知,我需要生成一个包含所有可能唯一组合的单个列表。 例如,给出以下列表:
X: [A, B, C]
Y: [W, X, Y, Z]
然后我应该能够生成12种组合:
[AW, AX, AY, AZ, BW, BX, BY, BZ, CW, CX, CY, CZ]
如果添加了3个元素的第三个列表,我将有36个组合,依此类推。
关于如何用Java做到这一点的任何想法?
(伪代码也没问题)
答案 0 :(得分:51)
你需要递归:
假设您的所有列表都在Lists
,这是一个列表列表。让结果成为所需排列的列表:这样做
void GeneratePermutations(List<List<Character>> Lists, List<String> result, int depth, String current)
{
if(depth == Lists.size())
{
result.add(current);
return;
}
for(int i = 0; i < Lists.get(depth).size(); ++i)
{
GeneratePermutations(Lists, result, depth + 1, current + Lists.get(depth).get(i));
}
}
最终的电话将是这样的
GeneratePermutations(Lists, Result, 0, EmptyString);
答案 1 :(得分:15)
这个话题派上了用场。我用Java完全重写了以前的解决方案,更加用户友好。此外,我使用集合和泛型来提高灵活性:
/**
* Combines several collections of elements and create permutations of all of them, taking one element from each
* collection, and keeping the same order in resultant lists as the one in original list of collections.
*
* <ul>Example
* <li>Input = { {a,b,c} , {1,2,3,4} }</li>
* <li>Output = { {a,1} , {a,2} , {a,3} , {a,4} , {b,1} , {b,2} , {b,3} , {b,4} , {c,1} , {c,2} , {c,3} , {c,4} }</li>
* </ul>
*
* @param collections Original list of collections which elements have to be combined.
* @return Resultant collection of lists with all permutations of original list.
*/
public static <T> Collection<List<T>> permutations(List<Collection<T>> collections) {
if (collections == null || collections.isEmpty()) {
return Collections.emptyList();
} else {
Collection<List<T>> res = Lists.newLinkedList();
permutationsImpl(collections, res, 0, new LinkedList<T>());
return res;
}
}
/** Recursive implementation for {@link #permutations(List, Collection)} */
private static <T> void permutationsImpl(List<Collection<T>> ori, Collection<List<T>> res, int d, List<T> current) {
// if depth equals number of original collections, final reached, add and return
if (d == ori.size()) {
res.add(current);
return;
}
// iterate from current collection and copy 'current' element N times, one for each element
Collection<T> currentCollection = ori.get(d);
for (T element : currentCollection) {
List<T> copy = Lists.newLinkedList(current);
copy.add(element);
permutationsImpl(ori, res, d + 1, copy);
}
}
我正在使用guava库来创建集合。
答案 2 :(得分:7)
此操作称为cartesian product。 Guava为其提供了实用功能:Lists.cartesianProduct
答案 3 :(得分:5)
没有递归唯一组合:
String sArray[] = new String []{"A", "A", "B", "C"};
//convert array to list
List<String> list1 = Arrays.asList(sArray);
List<String> list2 = Arrays.asList(sArray);
List<String> list3 = Arrays.asList(sArray);
LinkedList<List <String>> lists = new LinkedList<List <String>>();
lists.add(list1);
lists.add(list2);
lists.add(list3);
Set<String> combinations = new TreeSet<String>();
Set<String> newCombinations;
for (String s: lists.removeFirst())
combinations.add(s);
while (!lists.isEmpty()) {
List<String> next = lists.removeFirst();
newCombinations = new TreeSet<String>();
for (String s1: combinations)
for (String s2 : next)
newCombinations.add(s1 + s2);
combinations = newCombinations;
}
for (String s: combinations)
System.out.print(s+" ");
答案 4 :(得分:5)
为列表的通用列表List<List<T>>
添加基于迭代器的答案,从Ruslan Ostafiichuk的答案中扩展了这个想法。我遵循的想法是:
* List 1: [1 2]
* List 2: [4 5]
* List 3: [6 7]
*
* Take each element from list 1 and put each element
* in a separate list.
* combinations -> [ [1] [2] ]
*
* Set up something called newCombinations that will contains a list
* of list of integers
* Consider [1], then [2]
*
* Now, take the next list [4 5] and iterate over integers
* [1]
* add 4 -> [1 4]
* add to newCombinations -> [ [1 4] ]
* add 5 -> [1 5]
* add to newCombinations -> [ [1 4] [1 5] ]
*
* [2]
* add 4 -> [2 4]
* add to newCombinations -> [ [1 4] [1 5] [2 4] ]
* add 5 -> [2 5]
* add to newCombinations -> [ [1 4] [1 5] [2 4] [2 5] ]
*
* point combinations to newCombinations
* combinations now looks like -> [ [1 4] [1 5] [2 4] [2 5] ]
* Now, take the next list [6 7] and iterate over integers
* ....
* 6 will go into each of the lists
* [ [1 4 6] [1 5 6] [2 4 6] [2 5 6] ]
* 7 will go into each of the lists
* [ [1 4 6] [1 5 6] [2 4 6] [2 5 6] [1 4 7] [1 5 7] [2 4 7] [2 5 7]]
现在的代码。我使用Set
只是为了摆脱任何重复。可以用List
替换。一切都应该无缝地工作。 :)
public static <T> Set<List<T>> getCombinations(List<List<T>> lists) {
Set<List<T>> combinations = new HashSet<List<T>>();
Set<List<T>> newCombinations;
int index = 0;
// extract each of the integers in the first list
// and add each to ints as a new list
for(T i: lists.get(0)) {
List<T> newList = new ArrayList<T>();
newList.add(i);
combinations.add(newList);
}
index++;
while(index < lists.size()) {
List<T> nextList = lists.get(index);
newCombinations = new HashSet<List<T>>();
for(List<T> first: combinations) {
for(T second: nextList) {
List<T> newList = new ArrayList<T>();
newList.addAll(first);
newList.add(second);
newCombinations.add(newList);
}
}
combinations = newCombinations;
index++;
}
return combinations;
}
一个小试块..
public static void main(String[] args) {
List<Integer> l1 = Arrays.asList(1,2,3);
List<Integer> l2 = Arrays.asList(4,5);
List<Integer> l3 = Arrays.asList(6,7);
List<List<Integer>> lists = new ArrayList<List<Integer>>();
lists.add(l1);
lists.add(l2);
lists.add(l3);
Set<List<Integer>> combs = getCombinations(lists);
for(List<Integer> list : combs) {
System.out.println(list.toString());
}
}
答案 5 :(得分:3)
使用此处其他答案提供的嵌套循环解决方案来组合两个列表。
如果您有两个以上的列表,
答案 6 :(得分:2)
最后的类和public class TwoDimensionalCounter<T> {
private final List<List<T>> elements;
public TwoDimensionalCounter(List<List<T>> elements) {
this.elements = Collections.unmodifiableList(elements);
}
public List<T> get(int index) {
List<T> result = new ArrayList<>();
for(int i = elements.size() - 1; i >= 0; i--) {
List<T> counter = elements.get(i);
int counterSize = counter.size();
result.add(counter.get(index % counterSize));
index /= counterSize;
}
return result;//Collections.reverse() if you need the original order
}
public int size() {
int result = 1;
for(List<T> next: elements) result *= next.size();
return result;
}
public static void main(String[] args) {
TwoDimensionalCounter<Integer> counter = new TwoDimensionalCounter<>(
Arrays.asList(
Arrays.asList(1, 2, 3),
Arrays.asList(1, 2, 3),
Arrays.asList(1, 2, 3)
));
for(int i = 0; i < counter.size(); i++)
System.out.println(counter.get(i));
}
}
方法:
'KVG'
答案 7 :(得分:1)
使用 Java 8 Stream map
和 reduce
方法生成组合:
public static <T> List<List<T>> combinations(List<List<T>> lists) {
// incorrect incoming data
if (lists == null) return Collections.emptyList();
return lists.stream()
// non-null and non-empty lists
.filter(list -> list != null && list.size() > 0)
// represent each list element as a singleton list
.map(list -> list.stream().map(Collections::singletonList)
// Stream<List<List<T>>>
.collect(Collectors.toList()))
// summation of pairs of inner lists
.reduce((list1, list2) -> list1.stream()
// combinations of inner lists
.flatMap(inner1 -> list2.stream()
// merge two inner lists into one
.map(inner2 -> Stream.of(inner1, inner2)
.flatMap(List::stream)
.collect(Collectors.toList())))
// list of combinations
.collect(Collectors.toList()))
// otherwise an empty list
.orElse(Collections.emptyList());
}
public static void main(String[] args) {
List<String> list1 = Arrays.asList("A", "B", "C");
List<String> list2 = Arrays.asList("W", "X", "Y", "Z");
List<String> list3 = Arrays.asList("L", "M", "K");
List<List<String>> lists = Arrays.asList(list1, list2, list3);
List<List<String>> combinations = combinations(lists);
// column-wise output
int rows = 6;
IntStream.range(0, rows).forEach(i -> System.out.println(
IntStream.range(0, combinations.size())
.filter(j -> j % rows == i)
.mapToObj(j -> combinations.get(j).toString())
.collect(Collectors.joining(" "))));
}
按列输出:
[A, W, L] [A, Y, L] [B, W, L] [B, Y, L] [C, W, L] [C, Y, L]
[A, W, M] [A, Y, M] [B, W, M] [B, Y, M] [C, W, M] [C, Y, M]
[A, W, K] [A, Y, K] [B, W, K] [B, Y, K] [C, W, K] [C, Y, K]
[A, X, L] [A, Z, L] [B, X, L] [B, Z, L] [C, X, L] [C, Z, L]
[A, X, M] [A, Z, M] [B, X, M] [B, Z, M] [C, X, M] [C, Z, M]
[A, X, K] [A, Z, K] [B, X, K] [B, Z, K] [C, X, K] [C, Z, K]
答案 8 :(得分:0)
您需要实施的操作称为笛卡尔积。 有关详细信息,请参阅https://en.wikipedia.org/wiki/Cartesian_product
我建议使用我的开源库,它可以完全满足您的需求: https://github.com/SurpSG/Kombi
有一个例子如何使用它: https://github.com/SurpSG/Kombi#usage-for-lists-1
注意强>: 该库专为高性能目的而设计。您可以观察banchmarks结果here
该库为您提供了非常好的吞吐量和持续的内存使用
答案 9 :(得分:-5)
以下是使用位掩码的示例。没有递归和多个列表
static List<Integer> allComboMatch(List<Integer> numbers, int target) {
int sz = (int)Math.pow(2, numbers.size());
for (int i = 1; i < sz; i++) {
int sum = 0;
ArrayList<Integer> result = new ArrayList<Integer>();
for (int j = 0; j < numbers.size(); j++) {
int x = (i >> j) & 1;
if (x == 1) {
sum += numbers.get(j);
result.add(j);
}
}
if (sum == target) {
return result;
}
}
return null;
}