题解 | 好串

//这题跟括号配对问题是一样的，只是换成了ab配对单字符而已
import java.util.*;
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        Deque<Character> stack = new ArrayDeque<>();
        String s = in.next();
        stack.push('1');//防止'b'先手,导致误判
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == 'a' ) {
                stack.push(s.charAt(i));
                continue;
            }
            if (s.charAt(i) == 'b' && !stack.isEmpty()) {
                char c = stack.pop();
                if (c != 'a') {
                    System.out.println("Bad");
                    return;
                }
                continue;
            }
        }
        stack.pop();//清除加入'1'带来的影响
        if (!stack.isEmpty())
            System.out.println("Bad");
        else System.out.println("Good");
    }
}