P26.java

package com.shekhargulati.ninetynine_problems._01_lists;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
import java.util.stream.Stream;

import static java.util.stream.Collectors.toList;

/**
 * (**) Generate the combinations of K distinct objects chosen from the N elements of a list
 */
public class P26 {

    public static <T> List<List<T>> combinations(List<T> list, int k) {
        if (k == 0 || list.isEmpty()) {
            return Collections.emptyList();
        }
        if (k == 1) {
            return list.stream().map(e -> Stream.of(e).collect(toList())).collect(toList());
        }
        Map<Boolean, List<T>> headAndTail = P17.split(list, 1);
        List<T> head = headAndTail.get(true);
        List<T> tail = headAndTail.get(false);
        List<List<T>> c1 = combinations(tail, (k - 1)).stream().map(e -> {
            List<T> l = new ArrayList<>();
            l.addAll(head);
            l.addAll(e);
            return l;
        }).collect(Collectors.toList());
        List<List<T>> c2 = combinations(tail, k);
        c1.addAll(c2);
        return c1;
    }
}