dfs(depth)

N = 3, M = 2
입력: 3 1 2 → 정렬: [1, 2, 3]

dfs(0) 시작

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.*;

public class Back_15654 {
    static int N, M;
    static int[] input, output;
    static boolean[] visited;
    static StringBuilder sb = new StringBuilder();

    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        StringTokenizer st = new StringTokenizer(br.readLine());

        N = Integer.parseInt(st.nextToken());
        M = Integer.parseInt(st.nextToken());

        input = new int[N];
        output = new int[M];
        visited = new boolean[N];

        st = new StringTokenizer(br.readLine());
        for(int i = 0; i < N; i++) input[i] = Integer.parseInt(st.nextToken());

        Arrays.sort(input); // 오름차순 정렬
        dfs(0);
        System.out.print(sb);
    }

    static void dfs(int depth) {
        if (depth == M) {
            for (int i = 0; i < M; i++) sb.append(output[i]).append(" ");
            sb.append("\n");
            return;
        }

        for (int i = 0; i < N; i++) {
            if (!visited[i]) {
                visited[i] = true;
                output[depth] = input[i];
                dfs(depth + 1);
                visited[i] = false;
            }
        }
    }
}