K-th Beautiful String

solution.java

import java.util.*;
 
public class cf {
 
    public static void main(String[] args) {
        // Use the Scanner class
        Scanner sc = new Scanner(System.in);
        int n = Integer.parseInt(sc.nextLine());
         for (int i = 0; i < n; i++) {
             String s   = sc.nextLine();
             String[] result = s.split(" ");
             int a = Integer.parseInt(result[0]);
             int b = Integer.parseInt(result[1]);
 
             String str = "a";
             String repeated  = String.join("", Collections.nCopies(a, str));
             StringBuilder sb = new StringBuilder(repeated);
             if (b == 1) {
                 sb.setCharAt(repeated.length()-1,'b');
                 sb.setCharAt(repeated.length()-2,'b');
 
                 System.out.println(sb.toString());
             } else {
                 int sum = 1;
                 for (int j = 2; j < a; j++) {
                     sum = sum + j;
                     if (b <= (sum)) {
                         sb.setCharAt(repeated.length()-(j+1),'b');
                         sb.setCharAt(repeated.length()-(j-(sum-b)),'b');
//                     System.out.println("sum:" + sum + ", j: " + j + ", b:" + b);
                         System.out.println(sb.toString());
                         break;
                     }
                 }
             }
         }
    }
}