题解 | #24点运算#

       主要考回溯法求全排列     
``` import java.util.*;
public class TwenlyFourygame {
    public static List<String> opList=Arrays.asList("+","-","*","/");
    public static List<String> reList=new ArrayList<>();
    public static HashMap<String, Integer> map = 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);
        }
    };
    public static HashMap<Integer,String> mapReverse = new HashMap<Integer,String>(){
        {
            put( 1,"A");
            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( 11,"J");
            put( 12,"Q");
            put( 13,"K");
        }
    };
    public static int maxn=100;
    public static boolean flag=false;
    public static String[] opReList=new String[maxn];
    public static int[] a=new int[maxn];
    public static int[] b=new int[maxn];
    public static void main(String[] args){
        Scanner in = new Scanner(System.in);
        while(in.hasNext()){
            String s = in.nextLine();
            String[] arr = s.split(" ");
            boolean f=true;
            for(int i=0;i<arr.length;i++){
                if("joker".equals(arr[i].toLowerCase())){
                    f=false;
                    break;
                }else{
                    b[i]=map.get(arr[i]);
                }
            }
            if(!f){
                System.out.println("ERROR");
            }else{
                permutation(0,b);
                if(reList.size()!=0){
                    System.out.println(reList.get(0));
                }else{
                    System.out.println("NONE");
                }
            }

            reList.clear();
        }
    }
    public static void permutation(int cur ,int[] p){
        if(cur==4){
            dfs(0,0);

        }else for(int i=0;i<4;i++){
            int c1=0,c2=0;
            for(int j=0;j<cur;j++)if(a[j]==p[i])c1++;
            for(int j=0;j<4;j++)if(p[j]==p[i])c2++;
            if(c1<c2){
                a[cur]=p[i];
                permutation(cur+1,p);
            }
        }
    }
    public static void dfs(int cur ,int sum){
        if(cur==4){
            if(sum==24){
                if("+".equals(opReList[0])){
                    StringBuilder sb=new StringBuilder();
                    for(int i=0;i<3;i++){
                        sb.append(mapReverse.get(a[i]));
                        sb.append(opReList[i+1]);
                    }
                    sb.append(mapReverse.get(a[3]));
                    reList.add(sb.toString());
                }
                return;
            }
        }else for(int i=0;i<4;i++){
            String op=opList.get(i);
            switch(op){
                case "+" :
                    opReList[cur]="+";
                    dfs(cur+1,sum+a[cur]);
                    break;
                case "-" :
                    opReList[cur]="-";
                    dfs(cur+1,sum-a[cur]);
                    break;
                case "*" :
                    opReList[cur]="*";
                    dfs(cur+1,sum*a[cur]);
                    break;
                case "/" :
                    opReList[cur]="/";
                    dfs(cur+1,sum/a[cur]);
                    break;
            }

        }
    }
}