题解 | #24点游戏算法#

import java.util.HashSet;
import java.util.Scanner;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;

public class Main {
    private static HashSet<String> numsCombineSet = new HashSet<>();
    private static boolean can24 = false;
    private static boolean[] visited = new boolean[4];

    private static final String[] ops = new String[]{"+", "-", "*", "/"};

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

        while(in.hasNext()){
            int[] nums = new int[]{in.nextInt(), in.nextInt(), in.nextInt(), in.nextInt()};

            numsCombineSet = new HashSet<>();
            can24 = false;
            visited = new boolean[4];

            // solution(nums);
            solutionDFS(nums);
        }
    }

    /**
     * dfs
     * @param nums
     */
    private static void solutionDFS(int[] nums){

        dfs(nums, 0, 0d);

        if(can24){
            System.out.println("true");
        }else{
            System.out.println("false");
        }
    }

    private static boolean dfs(int[] nums, int opTimes, double result){
        if(opTimes==4 && result==24d){
            can24 = true;
            return true;
        }

        for(int i=0; i<4; i++){
            if(!visited[i]){
                visited[i] = true;

                if(opTimes == 0){
                    if(dfs(nums, opTimes+1, result+nums[i])){
                        return true;
                    }
                }else{
                    if(dfs(nums, opTimes+1, result+nums[i])
                            || dfs(nums, opTimes+1, result-nums[i])
                            || dfs(nums, opTimes+1, result*nums[i])
                            || dfs(nums, opTimes+1, result/nums[i])){
                        return true;
                    }
                }

                visited[i] = false;
            }
        }

        return false;
    }


    /**
     * 递归
     * @param nums
     * @throws ScriptException
     */
    private static void solution(int[] nums) throws ScriptException {
        // 生成各种数字组合
        numsCombine(nums, 0);

        for(String numsSeq: numsCombineSet){
            if(is24(numsSeq)){
                break;
            }
        }

        if(can24){
            System.out.println("true");
        }else{
            System.out.println("false");
        }
    }

    private static void numsCombine(int[] nums, int i) {

        StringBuilder sb = new StringBuilder();
        sb.append(nums[0]);
        for(int j=1; j<=3; j++){
            sb.append(",");
            sb.append(nums[j]);
        }
        numsCombineSet.add(sb.toString());

        for(int j=i; j<=3; j++){
            swap(nums, i, j);
            numsCombine(nums, i+1);
            swap(nums, i, j);
        }
    }

    private static boolean is24(String nums) throws ScriptException {
        String[] numsArray = nums.split(",");

        String opStr = numsArray[0];
        if(operate(numsArray, opStr, 1)){
            return true;
        }

        return false;
    }

    private static boolean operate(String[] nums, String opStr, int opTimes) throws ScriptException {
        if(opTimes == 4){
            ScriptEngine scriptEngine = new ScriptEngineManager().getEngineByName("nashorn");
            if("24".equals(String.valueOf(scriptEngine.eval(opStr)))){
                // 打印值为24的表达式
                System.out.println(opStr);
                can24 = true;
                return true;
            };
        }

        if(opTimes <= 3){
            for(int i=0; i<=3; i++){
                String tmpOpStr = "("+opStr+ops[i]+nums[opTimes]+")";
                opTimes++;
                if(operate(nums, tmpOpStr, opTimes)){
                    return true;
                }
                opTimes--;
            }
        }

        return false;
    }

    private static void swap(int[] nums, int i, int j){
        int tmp = nums[i];
        nums[i] = nums[j];
        nums[j] = tmp;
    }
}