Help me in solving DNASTORAGE problem

My issue

not able to understand the question please can anyone explain me the program in detail

My code

/* package codechef; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
class Codechef
{
	public static void main (String[] args) throws java.lang.Exception
	{
		Scanner sc = new Scanner(System.in);
		int t = sc.nextInt();
		for(int i=0 ; i <t ; i++){
		    int N = sc.nextInt();
		    String S = sc.next();
		    for(int j=0 ; j < N ; j = j+2){
		        if(S.charAt(j)== '0' && S.charAt(j+1) == '0'){
		           System.out.print("A"); 
		        }
		         if(S.charAt(j)== '0' && S.charAt(j+1) =='1'){
		           System.out.print("T"); 
		        }
		         if(S.charAt(j)== '1' && S.charAt(j+1) == '0'){
		           System.out.print("C"); 
		        }
		         if(S.charAt(j)== '1' && S.charAt(j+1) == '1'){
		           System.out.print("G"); 
		        }
		       
		    }
		    System.out.println();
		}
	}
}

Learning course: Strings using Java
Problem Link: CodeChef: Practical coding for everyone

@testo
its a simple implementation problem
U have to follow these rules while iterating through the string .

  • 00 is replaced with A
  • 01 is replaced with T
  • 10 is replaced with C
  • 11 is replaced with G