This passes all cases barr 5. Seems to be an issue with test case 5 as others have pointed out.

public class Solution {

 public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        int t = s.nextInt();
        String [] pair_left = new String[t];
        String [] pair_right = new String[t];
        
        for (int i = 0; i < t; i++) {
            pair_left[i] = s.next();
            pair_right[i] = s.next();
        }



        HashSet<String> strings = new HashSet<String>();

        // counter
        int lines = 0;


        for (int i = 0; i < t; i++){
            String concat = pair_left[i] + " " + pair_right[i];
            
            if(!strings.contains(concat)){
                strings.add(concat);
                lines++;
            }
            System.out.println(lines);
}

The problem statement clearly says "That also implies (a, b) is not same as (b, a)", but following that causes test case 5 to fail. To "fix" my solution, I had to treat (a, b) and (b, a) as identical. Please update the problem to reflect this.

Is it possible to get confirmation that test 5 contains a bug/ faulty test data? i spent ages looking through it but i cannot figure out whats wrong. Some people seem to have found solutions to pass all tests though

I think the test case 5 is wrong. Does anybody have the same opinion? I am totally confused...

Passed Test

import java.io.; import java.util.; import java.text.; import java.math.; import java.util.regex.*;

public class Solution {

public static void main(String[] args) { Scanner s = new Scanner(System.in); int t = s.nextInt(); String [] pair_left = new String[t]; String [] pair_right = new String[t];

    for (int i = 0; i < t; i++) {
        pair_left[i] = s.next();
        pair_right[i] = s.next();
    }
   // Use a HashSet to store unique pairs
    Set<String> uniquePairs = new HashSet<>();

    for (int i = 0; i < t; i++) {
        String[] pair = {pair_left[i], pair_right[i]};
        Arrays.sort(pair); // Sort the pair to ensure consistent ordering
        String sortedPair = pair[0] + "," + pair[1];
        uniquePairs.add(sortedPair);
        System.out.println(uniquePairs.size());
    }

} }