Help me in solving XYSTRP problem

bug reports
,
1

My issue

I’ve produced the correct result (with the sample input), but it says wrong answer.
I can clearly see the output matching sample output. Could this be some usabilty issue?

My code

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

class Codechef
{
	public static void main (String[] args) throws java.lang.Exception
	{
		Scanner scanner = new Scanner(System.in);

        Integer numTestCases = scanner.nextInt();
        List<String> list = new ArrayList<>();
        for (int i = 0; i < numTestCases; i++) {
            list.add(scanner.nextLine());
        }

        Stack<Character> stack = new Stack<>();

        list.forEach(str -> {
            int numPair = 0;
            for (int i = 0; i < str.length(); i++) {
                char current = str.charAt(i);

                if (stack.empty()) {
                    stack.push(current);
                } else {
                    char previous = stack.pop();
                    if (previous == current) {
                        stack.push(current);
                    } else {
                        numPair++;
                    }
                }
            }

            System.out.println(numPair);
        });
    }
}

Learning course: Greedy Algorithms
Problem Link: Chef and String Practice Problem in Greedy Algorithms - CodeChef