题解 | #24点运算#

//分别求4个数字的排序，操作符的排序 
//Ps:操作符可以重复
import java.util.*;

public class Main{
    private static HashMap<Character,Integer> map=new HashMap<>();
    private static HashMap<Integer,Character> map2=new HashMap<>();
    static{
            map.put('A', 1);map.put('J', 11);map.put('Q', 12);map.put('K', 13);map.put('-', 10);
            for(int i=2;i<=9;i++){
                map.put((char)('0'+i), i);
            }
    }
    static{
            map2.put(1,'A');map2.put(11,'J');map2.put(12,'Q');map2.put(13,'K');map2.put(10,'-');
            for(int i=2;i<=9;i++){
                map2.put(i,(char)('0'+i));
            }
    }
    public static void main(String[] args)throws Exception{
        Scanner in=new Scanner(System.in);
        Solution solution=new Solution();
        String s=in.nextLine();
        char[] nums=new char[4];
        String[] strs=s.split(" ");
        for(int i=0;i<4;i++){
            if(strs[i].equals("joker")||strs[i].equals("JOKER")){
                System.out.println("ERROR");
                return;
            }
            if(strs[i].equals("10")){
                nums[i]='-';
            }else
                nums[i]=strs[i].charAt(0);
        }
        ArrayList<ArrayList<Character>> res1=new ArrayList<>();
        ArrayList<ArrayList<Character>> res2=new ArrayList<>();
        solution.perm(nums, new Stack<Character>(), res1);
        solution.noperm(new char[]{'+','-','*','/'}, new Stack<Character>(), res2);
        for(int i=0;i<res1.size();i++){
            Stack<Integer> stack1=new Stack<>();
            for(int k=0;k<res1.get(i).size();k++){
                stack1.push(map.get(res1.get(i).get(k)));
            }
            for(int j=0;j<res2.size();j++){
                Stack<Character> stack2=new Stack<>();
                for(int m=0;m<res2.get(j).size();m++){
                    stack2.push(res2.get(j).get(m));
                }
                if(solution.core(stack1, stack2)){
                    //System.out.println(stack1+" "+stack2);
                    char a=map2.get(stack1.pop()), b=map2.get(stack1.pop()), c=map2.get(stack1.pop()), d=map2.get(stack1.pop());
                    char op1=stack2.pop(),op2=stack2.pop(),op3=stack2.pop();
                    System.out.println(""+a+op1+b+op2+c+op3+d);
                    return;
                }
            }
        }
        System.out.println("NONE");
        in.close();
    }
   
}
class Solution{
  //求值
    public boolean core(Stack<Integer> stack1,Stack<Character> stack2){
        //System.out.println(stack1+""+s2);
        Stack<Integer> s1=(Stack<Integer>)stack1.clone();
        Stack<Character> s2=(Stack<Character>)stack2.clone();
        //System.out.print(s1+""+s2+"=");
        while(!s2.isEmpty()){
            char op=s2.pop();
            int temp=0;
            int a=s1.pop();
            int b=s1.pop();
            if(op=='+'){
                temp=a+b;
            }
            if(op=='-'){
                temp=a-b;
            }
            if(op=='*'){
                temp=a*b;
            }
            if(op=='/'){
                temp=a/b;
            }
            s1.push(temp);
        }
        return s1.pop()==24?true:false;
    }
  //四个数全排列
    public void perm(char[] array,Stack<Character> stack,ArrayList<ArrayList<Character>> res){
        if(array.length<=0){
          res.add(new ArrayList<>(stack)); 
        }else{
            for(int i=0;i<array.length;i++){
                char[] temp=new char[array.length-1];
                System.arraycopy(array, 0, temp, 0, i);
                System.arraycopy(array,i+1, temp, i, array.length-i-1);
                stack.push(array[i]);
                perm(temp, stack,res);
                stack.pop();
            }
        }
    }
  //四种操作符
    public void noperm(char[] array,Stack<Character> stack,ArrayList<ArrayList<Character>> res){
        if(stack.size()==3){
            res.add(new ArrayList<>(stack));
        }else{
            for(int i=0;i<array.length;i++){
                stack.push(array[i]);
                noperm(array, stack, res);
                stack.pop();
            }
        }
        
    }
}