UVA - 468 - Key to Success

 import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Arrays;
import java.util.HashMap;

public class Main {

    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        StringBuffer sb = new StringBuffer("");
        String m="";
        int cases=Integer.parseInt(br.readLine());
        for (int i = 0; i < cases; i++) {
            br.readLine();
            if(i>0)
                sb.append("\n");
            m=br.readLine();
            charFreq []arr1=new charFreq[52];
            for(int j=0;j<26;j++){
                arr1[j]=new charFreq((char)('a'+j), 0);
                arr1[j+26]=new charFreq((char)('A'+j), 0);
            }
            for(int j=0;j<m.length();j++){
                if(m.charAt(j)>='a'&&m.charAt(j)<='z'){
                    arr1[m.charAt(j)-'a'].inc();
                }else if(m.charAt(j)>='A'&&m.charAt(j)<='Z'){
                    arr1[m.charAt(j)-'A'+26].inc();
                }
            }
            Arrays.sort(arr1);
            m=br.readLine();
            charFreq []arr2=new charFreq[52];
            for(int j=0;j<26;j++){
                arr2[j]=new charFreq((char)('a'+j), 0);
                arr2[j+26]=new charFreq((char)('A'+j), 0);
            }
            for(int j=0;j<m.length();j++){
                if(m.charAt(j)>='a'&&m.charAt(j)<='z'){
                    arr2[m.charAt(j)-'a'].inc();
                }else if(m.charAt(j)>='A'&&m.charAt(j)<='Z'){
                    arr2[m.charAt(j)-'A'+26].inc();
                }
            }
            Arrays.sort(arr2);
            HashMap<Integer,Integer> hm=new HashMap<Integer, Integer>();
            for(int j=arr1.length-1;j>-1;j--){
               hm.put((int)(arr2[j].getC()),(int) (arr1[j].getC()));
            }
            for(int j=0;j<m.length();j++){
                int temp=(hm.get((int)(m.charAt(j))));
                char x=(char) temp;
                sb.append(x);
            }
            sb.append("\n");
        }
        System.out.print(sb);
    }
}
   /*      
           
           
           
        }
       
    }
   
}*/
class charFreq implements Comparable<charFreq> {
    char c;
    int freq;

    public charFreq(char c, int freq) {
        this.c = c;
        this.freq = freq;
    }

    public char getC() {
        return c;
    }

    public int getFreq() {
        return freq;
    }
   
    public void inc(){
        freq++;
    }
    @Override
    public int compareTo(charFreq o) {
        if(this.freq>o.getFreq()){
            return 1;
        }
        return -1;
    }
   
}