1.

/*
 * ACM Contest training
 * Problem: 902 - Password Search
 * Link: http://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&category=11&problem=843
 *
 * @author Christoph Goettschkes
 * @version 1.0, 01/06/2011
 *
 * Method : Ad-Hoc
 * Status : Accepted
 * Runtime: 1.732
 */

import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;

class Main {

	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);

		while (input.hasNext()) {
			int pwLength = input.nextInt();
			String str = input.next();
			HashMap<String, Integer> m = new HashMap<String, Integer>();

			for (int i = 0; i < str.length() - pwLength + 1; i++) {
				String subs = str.substring(i, i + pwLength);
				if (m.containsKey(subs)) {
					m.put(subs, m.get(subs) + 1);
				} else {
					m.put(subs, 1);
				}
			}

			String ret = "";
			int max = Integer.MIN_VALUE;

			for (Map.Entry<String, Integer> k : m.entrySet()) {
				String key = k.getKey();
				int val = k.getValue();

				if (val > max) {
					max = val;
					ret = key;
				}
			}

			System.out.println(ret);
		}
	}
}

2. 

/**
 * FWP, Ausgewählte Probleme aus dem ACM Programming Contest, SS10 
 * Problem: 902 Password Search
 * Link: http://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&category=11&page=show_problem&problem=843
 * 
 * @author Siegfried Ippisch
 * @author Martin Lambeck
 * @version 1.0, 09/06/2010
 * 
 * Method : Strings
 * Status : Accepted
 * Runtime: 1.568
 */

import java.util.*;

public class Main {
	
	private static String searchPassword(int n, String text){
		Map<String, Integer> map = new HashMap<String, Integer>();
		for(int i=0; i<=text.length()-n; i++){
			String sub = text.substring(i, i+n);
			if(map.containsKey(sub))
				map.put(sub,map.get(sub)+1);
			else
				map.put(sub,1);
		}
		Map.Entry<String, Integer> result = null;
		for(Map.Entry<String, Integer> e: map.entrySet()){
			if(result == null || e.getValue() > result.getValue())
				result = e;
		}
		return result.getKey();
	}
	
	public static void main(String[] args){
		Scanner input = new Scanner(System.in);
		
		while(input.hasNext()){
			System.out.println(searchPassword(input.nextInt(),input.next()));
		}
		
	}
	
}