**********Змейка************ http://acmp.ru/index.asp?main=task&id_task=197

snake.java

import java.io.*;
import java.math.BigInteger;
import java.util.*;

public class Main{ 
    
  public static void main(String[] argv) throws IOException,Exception{
    new Main().run();
  }
 
  public static StreamTokenizer sc ; 
  public static int[][] arr; 
  public static int n; 
  public static int nextInt() throws Exception{ 
    sc.nextToken(); 
    return (int)sc.nval; 
  } 

  public void run() throws IOException,Exception{
    sc = new StreamTokenizer(new BufferedReader(new FileReader("input.txt")));   
    n = nextInt(); 
    int dx, dy, n2, x, y, nx, ny, temp, line;
    n2 = (int)Math.pow(n, 2);
    dy = 1;
    dx = -1;
    x = y = 0;
    line = 1;
    arr = new int[n][n];
    
    for (int i = 1; i < n2+1; i++) {
        arr[y][x] = i;
        nx = x + dx;
        ny = y + dy; 
        if (isValid(nx) && isValid(ny) && arr[ny][nx] == 0) {
            x = nx;
            y = ny;
        }else{
            if (line % 2 == 0) {
                if (isValid(x+1)) {
                    x++;
                }else if (isValid(y+1)){
                        y++;
                }
            }else {
                if (isValid(y+1)) {
                    y++;
                }else if (isValid(x+1)){
                        x++;
                }
            }   
            temp = dx;
            dx = dy;
            dy = temp;
            line ++;
        }
    }
    
    PrintWriter pw = new PrintWriter(new File("output.txt"));
    for (int[] row : arr) {
      for (int elem : row) {
        pw.printf("%6d", elem);
      }
      pw.println();
    }
    pw.close();
  }
  
  public static Boolean isValid(int i) {
      if (i >= 0 && i < n) {
          return true;
      }
      return false;
  }
}