题解 | 好串-Java

import java.util.Scanner;
import java.util.Stack;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String str = in.next();

        boolean result = goodString(str);
        if (result) System.out.print("Good");
        else System.out.print("Bad");
    }

    public static boolean goodString(String str) {
        Stack<Character> stack = new Stack<>();
        for (int i = 0; i < str.length(); i++) {
            char c = str.charAt(i);
            if (c == 'a') stack.push(c);
            else if (c == 'b') {
                // 当 Stack 为空时调用 pop() 方法，不会返回任何值，而是会直接抛出空栈异常
                if (stack.isEmpty()) return false;
                if (stack.pop() == 'b') return false;
            }
        }
        return stack.isEmpty();
    }
}