JAVA AC代码:

import java.util.*;
import java.util.StringTokenizer;
import java.io.*;

public class Main {

	public static void main(String[] args) {
		Stack<Integer> stack=new Stack<>();//掌握util库调用Stack(栈)
		int m=in.nextInt();
		while(m-->0){
			String op=in.next();
			if(op.equals("push")){
				int x=in.nextInt();
				stack.push(x);
			}else if(op.equals("pop")){
				if(!stack.isEmpty()) {
					stack.pop();
				}
			}else if(op.equals("empty")) {
				if(stack.isEmpty()) out.println("YES");
				else out.println("NO");
			}else {
				if(stack.isEmpty()) out.println("empty");
				else out.println(stack.peek());
			}
		}
		out.close();

	}
	
	

	static FastReader in=new FastReader();
	static PrintWriter out=new PrintWriter(System.out);
	static class FastReader{
		static BufferedReader br;
		static StringTokenizer st;
		FastReader(){
			br=new BufferedReader(new InputStreamReader(System.in));
		}
		String next() {
			String str="";
			while(st==null||!st.hasMoreElements()) {
				try {
					str=br.readLine();
				}catch(IOException e) {
					throw new RuntimeException(e);
				}
				st=new StringTokenizer(str);
			}
			return st.nextToken();
		}
		int nextInt() {
			return Integer.parseInt(next());
		}
		double nextDouble() {
			return Double.parseDouble(next());
		}
		long nextLong() {
			return Long.parseLong(next());
		}
	}
	
}