package com.thealgorithms.backtracking;
import java.util.ArrayList;
import java.util.List;
public final class ArrayCombination {
private ArrayCombination() {
}
public static List<List<Integer>> combination(int n, int k) {
if (n < 0 || k < 0 || k > n) {
throw new IllegalArgumentException("Wrong input.");
}
List<List<Integer>> combinations = new ArrayList<>();
combine(combinations, new ArrayList<>(), 0, n, k);
return combinations;
}
private static void combine(List<List<Integer>> combinations, List<Integer> current, int start, int n, int k) {
if (current.size() == k) {
combinations.add(new ArrayList<>(current));
return;
}
for (int i = start; i < n; i++) {
current.add(i);
combine(combinations, current, i + 1, n, k);
current.removeLast();
}
}
}