活字印刷

// 思路：使用回溯法与Java Stream API结合。首先计算每种排列组合，并将其收集到一个集合中，最后计算集合的大小。

import java.util.HashSet;
import java.util.Set;
import java.util.stream.IntStream;

public class TilePossibilitiesStream {

    public int numTilePossibilities(String tiles) {
        Set<String> result = new HashSet<>();
        boolean[] used = new boolean[tiles.length()];
        backtrack(tiles, used, new StringBuilder(), result);
        return result.size();
    }

    private void backtrack(String tiles, boolean[] used, StringBuilder current, Set<String> result) {
        if (current.length() > 0) {
            result.add(current.toString());
        }
        IntStream.range(0, tiles.length()).filter(i -> !used[i]).forEach(i -> {
            used[i] = true;
            current.append(tiles.charAt(i));
            backtrack(tiles, used, current, result);
            current.deleteCharAt(current.length() - 1);
            used[i] = false;
        });
    }

    public static void main(String[] args) {
        TilePossibilitiesStream solution = new TilePossibilitiesStream();
        System.out.println(solution.numTilePossibilities("AAB")); // 输出 8
    }
}