diff --git a/src/main/java/com/thealgorithms/backtracking/SubsequenceFinder.java b/src/main/java/com/thealgorithms/backtracking/SubsequenceFinder.java new file mode 100644 index 000000000000..4a159dbfe0b1 --- /dev/null +++ b/src/main/java/com/thealgorithms/backtracking/SubsequenceFinder.java @@ -0,0 +1,54 @@ +package com.thealgorithms.backtracking; + +import java.util.ArrayList; +import java.util.List; + +/** + * Class generates all subsequences for a given list of elements using backtracking + */ +public final class SubsequenceFinder { + private SubsequenceFinder() { + } + + /** + * Find all subsequences of given list using backtracking + * + * @param sequence a list of items on the basis of which we need to generate all subsequences + * @param the type of elements in the array + * @return a list of all subsequences + */ + public static List> generateAll(List sequence) { + List> allSubSequences = new ArrayList<>(); + if (sequence.isEmpty()) { + allSubSequences.add(new ArrayList<>()); + return allSubSequences; + } + List currentSubsequence = new ArrayList<>(); + backtrack(sequence, currentSubsequence, 0, allSubSequences); + return allSubSequences; + } + + /** + * Iterate through each branch of states + * We know that each state has exactly two branching + * It terminates when it reaches the end of the given sequence + * + * @param sequence all elements + * @param currentSubsequence current subsequence + * @param index current index + * @param allSubSequences contains all sequences + * @param the type of elements which we generate + */ + private static void backtrack(List sequence, List currentSubsequence, final int index, List> allSubSequences) { + assert index <= sequence.size(); + if (index == sequence.size()) { + allSubSequences.add(new ArrayList<>(currentSubsequence)); + return; + } + + backtrack(sequence, currentSubsequence, index + 1, allSubSequences); + currentSubsequence.add(sequence.get(index)); + backtrack(sequence, currentSubsequence, index + 1, allSubSequences); + currentSubsequence.removeLast(); + } +} diff --git a/src/test/java/com/thealgorithms/backtracking/SubsequenceFinderTest.java b/src/test/java/com/thealgorithms/backtracking/SubsequenceFinderTest.java new file mode 100644 index 000000000000..dac2e2675674 --- /dev/null +++ b/src/test/java/com/thealgorithms/backtracking/SubsequenceFinderTest.java @@ -0,0 +1,28 @@ +package com.thealgorithms.backtracking; + +import static org.junit.jupiter.api.Assertions.assertIterableEquals; + +import java.util.ArrayList; +import java.util.List; +import java.util.stream.Stream; +import org.junit.jupiter.params.ParameterizedTest; +import org.junit.jupiter.params.provider.MethodSource; + +public class SubsequenceFinderTest { + + @ParameterizedTest + @MethodSource("getTestCases") + void testGenerateAll(TestCase testData) { + final var actual = SubsequenceFinder.generateAll(testData.input()); + assertIterableEquals(testData.expected(), actual); + } + + static Stream getTestCases() { + return Stream.of(new TestCase(new ArrayList<>(), List.of(List.of())), new TestCase(List.of(1, 2), List.of(List.of(), List.of(2), List.of(1), List.of(1, 2))), + new TestCase(List.of("A", "B", "C"), List.of(List.of(), List.of("C"), List.of("B"), List.of("B", "C"), List.of("A"), List.of("A", "C"), List.of("A", "B"), List.of("A", "B", "C"))), + new TestCase(List.of(1, 2, 3), List.of(List.of(), List.of(3), List.of(2), List.of(2, 3), List.of(1), List.of(1, 3), List.of(1, 2), List.of(1, 2, 3))), new TestCase(List.of(2, 2), List.of(List.of(), List.of(2), List.of(2), List.of(2, 2)))); + } + + record TestCase(List input, List> expected) { + } +}