Sparse Arrays HackerRank

1.USING ARRAYLIST

import java.io.*;

import java.math.*;

import java.security.*;

import java.text.*;

import java.util.*;

import java.util.concurrent.*;

import java.util.function.*;

import java.util.regex.*;

import java.util.stream.*;

import static java.util.stream.Collectors.joining;

import static java.util.stream.Collectors.toList;

public class Solution {

    public static void main(String[] args) throws IOException {

        Scanner sc=new Scanner(System.in);

        int n=sc.nextInt();

        String str[]=new String[n];

        ArrayList<String> aa=new ArrayList<String>();

        

        for(int i=0;i<n;i++){

            str[i]=sc.next();

            aa.add(str[i]);

        }

        

        

        int q=sc.nextInt();

        String qry[]=new String[q];

        for(int i=0;i<q;i++){

            qry[i]=sc.next();

        }

        

        int index=0;

        int f[]=new int[qry.length];

        

        for(int i=0;i<q;i++){

            

            f[index++]=Collections.frequency(aa,qry[i]);

            

        }

        

        for(int i=0;i<f.length;i++){

            System.out.println(f[i]+" ");

        }

              

    }

}

/*TIME O(N)

SPACE O(N) using additional array and converting given string array to an ArrayList.

We add elements of a given string array to an ArrayList.

Then we iterate a query loop and store the frequency of each element in the f array

after seeing its occurrence in the ArrayList.

*/

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2.USING HASHMAP

import java.io.*;

import java.math.*;

import java.security.*;

import java.text.*;

import java.util.*;

import java.util.concurrent.*;

import java.util.function.*;

import java.util.regex.*;

import java.util.stream.*;

import static java.util.stream.Collectors.joining;

import static java.util.stream.Collectors.toList;

public class Solution {

    public static void main(String[] args) throws IOException {

        Scanner sc=new Scanner(System.in);

        int n=sc.nextInt();

        String str[]=new String[n];

        HashMap<String,Integer> aa=new HashMap<String,Integer>();

        

        for(int i=0;i<n;i++){

            str[i]=sc.next();

            

        }

        

        for(int i=0;i<n;i++){

            if(aa.containsKey(str[i])){

                aa.put(str[i],aa.get(str[i])+1);

            }

            else

                aa.put(str[i],1);

        }

            

        

        int q=sc.nextInt();

        String qry[]=new String[q];

        for(int i=0;i<q;i++){

            qry[i]=sc.next();

        }

        

        int index=0;

        int f[]=new int[qry.length];

        

        for(int i=0;i<q;i++){

            if(aa.containsKey(qry[i])){

                f[index++]=aa.get(qry[i]);

            }

            else{

                f[index++]=0;

            }

        }

        

        for(int i=0;i<f.length;i++){

            System.out.println(f[i]);

        }

        

    }

}

/*TIME O(N)

SPACE O(N) using hashmap

We will first store the string(from the string array) as key and their frequency as values in the hashmap.

Then we will iterate the query array and if we found a match with the hashmap key element

then we will store the value(which is the frequency of key ) in the f array.

*/