Language: Java

package Atcoder.panasonic2020;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.Serializable;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Stack;
import java.util.StringTokenizer;

/**
 * for (int i = 0; i < N; i++) {}
 * System.out.println();
 * HashMap<Integer, Integer> hm = new HashMap<>();
 * int[] s = new int[M];
 * ArrayDeque<Integer> deq = new ArrayDeque<>();
 *
 */
public class Main4 implements Runnable { //クラス名はMain4

	public static void main(String[] args) {
		Thread.setDefaultUncaughtExceptionHandler((t, e) -> System.exit(1));
		new Thread(null, new Main4(), "", 16 * 1024 * 1024).start(); //16MBスタックを確保して実行
	}

	public void run() {
		FastScannerMain4 sc = new FastScannerMain4(System.in);

		try {
			//String S = sc.nextLine().trim();
			int N = sc.nextInt();

			/*
			char s = (char) ('a');
			
			for (char ch = 'a'; ch <= 'j'; ch++) {
				System.out.println(ch);
			}
			*/

			dfs(N, (char) ('a'), "");

		} catch (Exception e) {
			e.printStackTrace();
		}

	}

	private void dfs(int N, char max, String ans) {

		if (ans.length() == N) {
			System.out.println(ans);
		} else {
			for (char ch = 'a'; ch <= max; ch++) {
				//System.out.println("  ch>" + ch + " max>" + max);
				dfs(N, (char) ((ch == max) ? max + 1 : max), ans + ch);
			}
		}
	}

	public static List<List<String>> combination(List list, int count) {
		List ret = new ArrayList<List<String>>();
		for (int i = 0; i < list.size(); i++) {
			if (i + count > list.size()) {
				break;
			}
			Stack stack = new Stack<String>();
			stack.push(list.get(i));
			_combination(ret, list, stack, i + 1, count);
		}
		return ret;
	}

	private static void _combination(List ret, List list, Stack stack, int index, int count) {
		for (int i = index; i < list.size(); i++) {
			stack.push(list.get(i));
			if (stack.size() == count) {
				ret.add(Arrays.asList(stack.toArray()));
				stack.pop();
				continue;
			}
			_combination(ret, list, stack, i + 1, count);
			stack.pop();
		}
	}
}

/**
*
* How to call
 Permutation<String> p = new Permutation<>(new String[] { "a", "b", "c" });
 do {
 	System.out.println(Arrays.toString(p.getObjes()));
 } while (p.next());

	PermutationWarshallFloyd<Integer> p = new PermutationWarshallFloyd<>(travels);
	do {
       //System.out.println(Arrays.toString(p.getObjes()));
       Integer[] per = p.getObjes();
       long eachans = 0;
       for (int i = 0; i < per.length; i++) {
           if (i > 0) {
       eachans += wf.distance(per[i - 1] - 1, per[i] - 1);
           }
       }
	} while (p.next());
*
*
*
* @author daichi.miyajima
*
* @param <T>
*/
class Permutation<T extends Serializable> {

	private int baseIndex;
	private int index;
	private T[] objs;

	private Permutation<T> subPermutation;

	public Permutation(T[] objs) {
		this(0, 0, objs.clone());
	}

	private Permutation(int baseIndex, int index, T[] objs) {
		if (objs == null || objs.length == 0) {
			throw new IllegalArgumentException();
		}

		this.baseIndex = baseIndex;
		this.index = index;
		this.objs = objs;

		if (this.index < this.objs.length - 1) {
			this.subPermutation = new Permutation<T>(this.baseIndex + 1, this.index + 1, this.objs);
		}
	}

	public T[] getTarget() {
		return this.objs;
	}

	public boolean next() {
		if (this.subPermutation == null) {
			return false;
		}

		boolean result = this.subPermutation.next();
		if (result == true) {
			return true;
		}

		this.swap(this.baseIndex, this.index);

		++this.index;
		if (this.objs.length <= this.index) {
			this.index = this.baseIndex;
			return false;
		}

		this.swap(this.index, this.baseIndex);
		return true;
	}

	public T[] getObjes() {
		return this.objs;
	}

	private void swap(int index1, int index2) {
		T tmp = this.objs[index1];
		this.objs[index1] = this.objs[index2];
		this.objs[index2] = tmp;
	}
}

//高速なScanner
class FastScannerMain4 {
	private BufferedReader reader = null;
	private StringTokenizer tokenizer = null;

	public FastScannerMain4(InputStream in) {
		reader = new BufferedReader(new InputStreamReader(in));
		tokenizer = null;
	}

	public String next() {
		if (tokenizer == null || !tokenizer.hasMoreTokens()) {
			try {
				tokenizer = new StringTokenizer(reader.readLine());
			} catch (IOException e) {
				throw new RuntimeException(e);
			}
		}
		return tokenizer.nextToken();
	}

	public String nextLine() {
		if (tokenizer == null || !tokenizer.hasMoreTokens()) {
			try {
				return reader.readLine();
			} catch (IOException e) {
				throw new RuntimeException(e);
			}
		}
		return tokenizer.nextToken("\n");
	}

	public long nextLong() {
		return Long.parseLong(next());
	}

	public int nextInt() {
		return Integer.parseInt(next());
	}

	public double nextDouble() {
		return Double.parseDouble(next());
	}

	public int[] nextIntArray(int n) {
		int[] a = new int[n];
		for (int i = 0; i < n; i++)
			a[i] = nextInt();
		return a;
	}

	public long[] nextLongArray(int n) {
		long[] a = new long[n];
		for (int i = 0; i < n; i++)
			a[i] = nextLong();
		return a;
	}
}


Compiler output:
Main.java:22: error: class Main4 is public, should be declared in a file named Main4.java
public class Main4 implements Runnable { //?????Main4
       ^
Note: Main.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
1 error