Combinations

Main.java

package com.asb.algos.playground;

import java.util.Arrays;
import java.util.Set;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class Main {

    public static void main(String[] args) throws Exception {
        CombinationGenerator cg = new CombinationGenerator();
        cg.generateCombinations("abbcde", 3)
                .forEach(s -> System.out.println(s));
    }

    public static class CombinationGenerator {

        public Set<String> generateCombinations(String s, int number)
                throws Exception {
            if (null == s || number > s.length() || number < 0) {
                throw new Exception("Invalid input");
            }
            char []str = s.toCharArray();
            Arrays.sort(str);
            return combination(String.copyValueOf(str), number);
        }

        private Set<String> combination(String s, int number) throws Exception {
            if (1 == number) {
                return IntStream
                        .range(0, s.length())
                        .mapToObj(i -> s.substring(i, i + 1))
                        .collect(Collectors.toSet());
            }
            return combination(s, number - 1)
                    .stream()
                    .flatMap(c -> IntStream
                            .range(s.lastIndexOf(c.charAt(c.length() - 1)) + 1,
                                    s.length())
                            .mapToObj(i -> c + s.substring(i, i + 1)))
                    .collect(Collectors.toSet());
        }
    }
}