Remove All Adjacent Duplicates

/*
 * Problem : Process text.
 * Input   : One line of text.
 * Output  : The reduced text.
 * Approach: Use a stack.
 * Time    : O(n)
 * Space   : O(n)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
    // This method solves the task. It receives the input and returns the answer.
    static String solve(String s) {
        // Step 1: A StringBuilder works like a stack. Its end is the top of the stack.
        StringBuilder stack = new StringBuilder();

        for (int i = 0; i < s.length(); i++) {
            char letter = s.charAt(i);

            // Step 2: If the top letter equals the new letter, they cancel each other. Remove the top.
            if (stack.length() > 0 && stack.charAt(stack.length() - 1) == letter) {
                stack.deleteCharAt(stack.length() - 1);
            } else {
                // Step 3: Otherwise keep the new letter.
                stack.append(letter);
            }
        }
        return stack.length() == 0 ? "empty" : stack.toString();
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String s = scanner.hasNextLine() ? scanner.nextLine() : "";
        System.out.println(solve(s));
    }
}