Language: Java

import java.io.*;
import java.util.*;
import java.math.BigInteger;
public class java{
  public static boolean[] visited;
  public static ArrayList<Integer>[] graph;
  public static final int MOD = (int) 1e9 + 7;
  public static void main(String[] args) {
    InputReader in = new InputReader(System.in);
    PrintWriter w = new PrintWriter(System.out);
    int n=in.nextInt(),m=in.nextInt();
    visited=new boolean[n];
    graph=new ArrayList[n];
    for(int i=0;i<n;i++){
      graph[i]=new ArrayList<>();
    }
    while (m-->0){
      int u=in.nextInt()-1;
      int v=in.nextInt()-1;
      graph[u].add(v);
      graph[v].add(u);
    }
    visited[0]=true;
    w.println(dfs(0));
    w.close();
    }
  static int dfs(int v){

    if (allvisited())
      return 1;
    else{
      int r=0;
      for(Integer u:graph[v]){
        if (!visited[u]){
          visited[u]=true;
          r=r+dfs(u);
          visited[u]=false;
        }

      }
      return r;
    }
  }
  static boolean allvisited(){
    for(int i=0;i<visited.length;i++){
      if (!visited[i])
        return false;
      }
    return true;
  }
  static class InputReader {

    private final InputStream stream;
    private final byte[] buf = new byte[8192];
    private int curChar, snumChars;
    private SpaceCharFilter filter;

    public InputReader(InputStream stream) {
      this.stream = stream;
    }

    public int snext() {
      if (snumChars == -1)
      throw new InputMismatchException();
      if (curChar >= snumChars) {
        curChar = 0;
        try {
          snumChars = stream.read(buf);
        } catch (IOException e) {
          throw new InputMismatchException();
        }
        if (snumChars <= 0)
        return -1;
      }
      return buf[curChar++];
    }

    public int nextInt() {
      int c = snext();
      while (isSpaceChar(c)) {
        c = snext();
      }
      int sgn = 1;
      if (c == '-') {
        sgn = -1;
        c = snext();
      }
      int res = 0;
      do {
        if (c < '0' || c > '9')
        throw new InputMismatchException();
        res *= 10;
        res += c - '0';
        c = snext();
      } while (!isSpaceChar(c));
      return res * sgn;
    }

    public long nextLong() {
      int c = snext();
      while (isSpaceChar(c)) {
        c = snext();
      }
      int sgn = 1;
      if (c == '-') {
        sgn = -1;
        c = snext();
      }
      long res = 0;
      do {
        if (c < '0' || c > '9')
        throw new InputMismatchException();
        res *= 10;
        res += c - '0';
        c = snext();
      } while (!isSpaceChar(c));
      return res * sgn;
    }

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

    public String readString() {
      int c = snext();
      while (isSpaceChar(c)) {
        c = snext();
      }
      StringBuilder res = new StringBuilder();
      do {
        res.appendCodePoint(c);
        c = snext();
      } while (!isSpaceChar(c));
      return res.toString();
    }

    public String nextLine() {
      int c = snext();
      while (isSpaceChar(c))
      c = snext();
      StringBuilder res = new StringBuilder();
      do {
        res.appendCodePoint(c);
        c = snext();
      } while (!isEndOfLine(c));
      return res.toString();
    }

    public boolean isSpaceChar(int c) {
      if (filter != null)
      return filter.isSpaceChar(c);
      return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
    }

    private boolean isEndOfLine(int c) {
      return c == '\n' || c == '\r' || c == -1;
    }

    public interface SpaceCharFilter {
      public boolean isSpaceChar(int ch);
    }
  }
}


Compiler output:
Main.java:4: error: class java is public, should be declared in a file named java.java
public class java{
       ^
Note: Main.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
1 error