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Combinatorics.java
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Combinatorics.java
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import java.util.*;
public class Combinatorics {
public static Iterable<int[]> combinations(int n, int k) {
if (k > n || n < 0 || k < 0) {
throw new IllegalArgumentException("n = " + n + ", k = " + k);
}
if (k == 0) {
return Collections.singleton(new int[0]);
}
return new CombinationsIterable(n, k);
}
static class CombinationsIterable implements Iterable<int[]> {
int n, k;
public CombinationsIterable(int n, int k) {
this.n = n;
this.k = k;
}
@Override
public Iterator<int[]> iterator() {
return new CombintationsIterator();
}
class CombintationsIterator implements Iterator<int[]> {
int[] a;
public CombintationsIterator() {
this.a = new int[k];
for (int i = 0; i < k; i++) {
a[i] = i;
}
a[k - 1]--;
}
@Override
public boolean hasNext() {
for (int i = k - 1; i >= 0; i--) {
if (a[i] != n - k + i) {
return true;
}
}
return false;
}
@Override
public int[] next() {
int i = k - 1;
while (i >= 0 && a[i] == n - k + i) {
i--;
}
if (i < 0) {
throw new NoSuchElementException();
}
a[i]++;
for (i++; i < k; i++) {
a[i] = a[i - 1] + 1;
}
return a.clone();
}
@Override
public void remove() {
throw new UnsupportedOperationException();
}
}
}
public static void main(String[] args) {
for (int[] a : combinations(5, 0)) {
System.out.println(Arrays.toString(a));
}
}
}