二十四点

好长时间没写Java了，用java水一道模拟题。哈希表里面存的是优先级，弹栈时要注意，假如是a/b弹栈弹出的先是b再是a，注意calc函数里面的’-‘跟’/’。

import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;
import java.util.Stack;

public class Main {

	private static Map<Character, Integer> m = new HashMap<Character, Integer>();
	static Stack<Integer> si = new Stack<Integer>();
	static Stack<Character> sc = new Stack<Character>();

	public static void main(String[] args) {
		m.put('+', 1);
		m.put('-', 1);
		m.put('x', 2);
		m.put('/', 2);
		int n = 0;
		Scanner sc = new Scanner(System.in);
		n = sc.nextInt();
		sc.nextLine();
		for (int i = 0; i < n; i++) {
			String line = sc.nextLine();
			int res = fun(line);
			if (res == 24)
				System.out.println("Yes");
			else
				System.out.println("No");
		}
		sc.close();
	}

	private static int fun(String line) {
		for (int i = 0; i < line.length(); i++) {
			if (line.charAt(i) < '0' || line.charAt(i) > '9') {
				if (sc.size() > 0 && m.get(line.charAt(i)) <= m.get(sc.peek())) 
					si.push(calc());
				sc.push(line.charAt(i));

			} else
				si.add(line.charAt(i) - '0');
		}
		while(!sc.isEmpty())si.add(calc());
		return si.pop();
	}

	private static int calc() {
		int a = si.pop();
		int b = si.pop();
		char op = sc.pop();
		if(op=='+')return a+b;
		else if(op=='-')return b-a;
		else if(op=='x')return a*b;
		return b/a;
	}

}