字母组合迭代器

/*
 * 使用Java Stream实现组合迭代器。
 * 
 * 思路：
 * 1. 使用递归生成所有可能的组合。
 * 2. 将生成的组合转换为流。
 * 3. 使用迭代器遍历流中的元素。
 */

import java.util.*;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class CombinationIterator {
    private Iterator<String> iterator;

    public CombinationIterator(String characters, int combinationLength) {
        List<String> combinations = new ArrayList<>();
        generateCombinations(characters, combinationLength, 0, new StringBuilder(), combinations);
        iterator = combinations.iterator();
    }

    private void generateCombinations(String characters, int combinationLength, int start, StringBuilder current, List<String> combinations) {
        if (current.length() == combinationLength) {
            combinations.add(current.toString());
            return;
        }
        for (int i = start; i < characters.length(); i++) {
            current.append(characters.charAt(i));
            generateCombinations(characters, combinationLength, i + 1, current, combinations);
            current.deleteCharAt(current.length() - 1);
        }
    }

    public String next() {
        return iterator.next();
    }

    public boolean hasNext() {
        return iterator.hasNext();
    }
}