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SelectionRecursiveCombinationGenerator.java
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SelectionRecursiveCombinationGenerator.java
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package com.baeldung.algorithms.combination;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class SelectionRecursiveCombinationGenerator {
private static final int N = 6;
private static final int R = 3;
/**
* Generate all combinations of r elements from a set
* @param n - number of elements in input set
* @param r - number of elements to be chosen
* @return the list containing all combinations
*/
public List<int[]> generate(int n, int r) {
List<int[]> combinations = new ArrayList<>();
helper(combinations, new int[r], 0, n - 1, 0);
return combinations;
}
/**
* Choose elements from set by recursing over elements selected
* @param combinations - List to store generated combinations
* @param data - current combination
* @param start - starting element of remaining set
* @param end - last element of remaining set
* @param index - number of elements chosen so far.
*/
private void helper(List<int[]> combinations, int data[], int start, int end, int index) {
if (index == data.length) {
int[] combination = data.clone();
combinations.add(combination);
} else {
int max = Math.min(end, end + 1 - data.length + index);
for (int i = start; i <= max; i++) {
data[index] = i;
helper(combinations, data, i + 1, end, index + 1);
}
}
}
public static void main(String[] args) {
SelectionRecursiveCombinationGenerator generator = new SelectionRecursiveCombinationGenerator();
List<int[]> combinations = generator.generate(N, R);
for (int[] combination : combinations) {
System.out.println(Arrays.toString(combination));
}
System.out.printf("generated %d combinations of %d items from %d ", combinations.size(), R, N);
}
}