题解 | 【模板】栈的操作

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
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) throws IOException {
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        StringBuilder out = new StringBuilder();
        int groups = Integer.parseInt(in.readLine());
        for (int i = 0; i < groups; i++) {
            int numOps = Integer.parseInt(in.readLine());
            String[] stack = new String[numOps];
            int top = 0;
            for (int j = 0; j < numOps; j++) {
                String[] strs = in.readLine().split(" ");
                switch (strs[0]) {
                    case "push":
                        stack[top] = (strs[1]);
                        top++;
                        break;
                    case "pop":
                        if (top >= 1) {
                            top--;
                        } else {
                            out.append("Empty\n");
                        }
                        break;
                    case "query":
                        if (top >= 1) {
                            out.append(stack[top - 1]).append('\n');
                        } else {
                            out.append("Anguei!\n");
                        }
                        break;
                    case "size":
                        out.append(top).append('\n');
                        break;
                }
            }
        }
        System.out.print(out);
    }
}