题解 | #24点运算#

import java.util.HashMap;
import java.util.Scanner;

public class Main {
    private static final HashMap<String, Integer> cardMap = new HashMap<String, Integer>(){{
        put("A", 1);
        put("2", 2);
        put("3", 3);
        put("4", 4);
        put("5", 5);
        put("6", 6);
        put("7", 7);
        put("8", 8);
        put("9", 9);
        put("10", 10);
        put("J", 11);
        put("Q", 12);
        put("K", 13);
    }};

    private static String[] cards = new String[4];
    private static boolean[] isVisited = new boolean[4];
    private static final char[] ops = new char[]{'+', '-', '*', '/'};

    private static boolean isComplete = false;
    private static String op24Str = "";


    public static void main(String[] args){
        Scanner in =  new Scanner(System.in);

        while(in.hasNext()){
            solution(in);
        }
    }

    /**
     * 模拟法
     * @param in
     */
    private static void solution(Scanner in){
        String line = in.nextLine();

        if(line.contains("joker") || line.contains("JOKER")){
            System.out.println("ERROR");
        }else{
            cards = line.split(" ");

            operate(0, 0, "");

            if(isComplete){
                System.out.println(op24Str);
            }else{
                System.out.println("NONE");
            }
        }
    }

    /**
     * 递归
     * @param opTimes
     * @param result
     * @param opStr
     */
    private static void operate(int opTimes, int result, String opStr){
        if(opTimes == 4){
            if(result == 24){
                isComplete = true;
                op24Str = opStr;
            }
            return;
        }

        // cards[i] - 4张卡牌
        for(int i=0; i<4; i++){
            if(isComplete){
                break;
            }
            // 卡牌是否已经使用
            if(!isVisited[i]){
                isVisited[i] = true;

                if(opTimes == 0){
                    int newResult = cardMap.get(cards[i]);
                    String newOpStr = cards[i];

                    operate(opTimes+1, newResult, newOpStr);
                }else{
                    // ops[j] - 加减乘除
                    for(int j=0; j<4; j++){
                        int newResult = 0;
                        switch(ops[j]){
                            case '+': newResult = result + cardMap.get(cards[i]); break;
                            case '-': newResult = result - cardMap.get(cards[i]); break;
                            case '*': newResult = result * cardMap.get(cards[i]); break;
                            case '/': newResult = result / cardMap.get(cards[i]); break;
                        }
                        String newOpStr = opStr + ops[j] + cards[i];

                        operate(opTimes+1, newResult, newOpStr);
                    }
                }

                isVisited[i] = false;
            }
        }
    }
}